TK 6550 
. H25 
1924 
Copy 2 



RADIO FREQUENCY 
AMPLIFICATION 


THEORY AND PRACTICE 


KENNETH HARKNESS 



























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RADIO FREQUENCY 
AMPLIFICATION 


THEORY AND PRACTICE 


BY 

KENNETH HARKNESS 



NEW YORK 

Stye iRaiito (Sutlb, Stic. 

Publishers 



y - ,, I 

SECOND EDITION 
Copyright 1924 
THE RADIO GUILD, INC. \ 

New York 

The entire contents of this book are copyrighted and must not 
be reproduced without the permission of the publishers. 




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CU79309S ^ 




Mmi -i 1924 


Printed in ^U. S. A. 


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INTRODUCTION 


While conducting the Radio Department of the Literary 
Digest, and in writing my book Practical Radio and the vol¬ 
ume on Radio in my Story of Modern Science, I have had occa¬ 
sion to confer with Mr. Kenneth Harkness many times and to 
quote from his admirably lucid and informative writings, par¬ 
ticularly with reference to the practicalities of receiving sets 
using the principle of Radio Frequency Amplification. 

Moreover, I have had his practical co-operation in the con¬ 
struction of such sets for my own use, and have had opportunity 
to witness at first hand the experiments through which Mr. 
Harkness has developed his quite unusual mastery of both prin¬ 
ciples and practise in a field until recently very little explored. 
It was, indeed, personal observation of, and tests with, instru¬ 
ments made by Mr. Harkness that confirmed my belief in the all¬ 
importance of the Radio Frequency method of reception—no¬ 
tably for the short-wave messages of the Broadcasting stations— 
at a time when most experts in this country looked askance at 
the method, or even openly decried it. 

It is indeed fortunate for the radio public that Mr. Harkness 
is a practised writer no less than a practical radio engineer. He 
cannot only do things, but can tell others how to do them. His 
book on Radio Frequency Amplification, standing practically 
alone in its field, should mark the beginning of a new era 
in radio reception for a host of eager listeners who have not 
hitherto been able to reach out to distant stations, but who now, 
under expert guidance, may reconstruct their sets with the ambi¬ 
tion to “tune in” at will, under favorable conditions, on the 
program of any high-power broadcasting station from Boston 
to Los Angeles and from Montreal to Havana. 

When you have that kind of a set on your desk, radio will 
take on new meanings. For the average amateur, who must 
work with relatively simple means, Radio Frequency alone offers 
such possibilities. And Kenneth Harkness is Radio Frequency's 
most ardent protagonist. 

Henry Smith Williams. 


V 





















. 































PREFACE TO THE FIRST EDITION 


I have often been interested to witness the reactions of a 
total stranger to “radio” when he first hears some event of cur¬ 
rent interest properly reproduced. 

A little incident which occurred recently may possibly in¬ 
terest the readers of this book. I made the acquaintance of a 
gentleman who was violently opposed to radio. We shall call 
him Mr. Conover. Mr. Conover’s opposition was mainly based 
on his impression that all radio receivers give forth the “horrible 
noises” which issue from the horns outside some radio stores 
and fill the neighborhood with a deafening uproar. We have 
all heard them. Unfortunately we cannot avoid hearing them. 
The stentorian voice of the announcer shakes the windows across 
the street. The modest strains of a violin are thrown out of 
the horn with an ear-splitting blast of triumph. A harmless 
little crackle of static rends the air like a bolt of thunder. Should 
a soprano sing, the air is filled with the piercing shriek of a 
steam whistle. This strange form of so-called advertising has 
successfully prevented thousands of people from possessing the 
slightest desire to own a radio receiver. 

These blatant distortions of radio reproduction had dinned 
in the ears of my friend Mr. Conover and followed him for 
blocks as he passed through the streets of the city. Each time 
he passed a radio store he made a mental reservation that he 
would never permit his young son, who threatened to take up 
radio, to transform his home into a madhouse by the installation 
of any apparatus which was capable of creating such an up¬ 
roarious racket. 

When this gentleman learned that I was engaged in the 
manufacture of radio receivers there was a distinct feeling of 
coldness between us. I could see that he regarded me as a de¬ 
cided pest. I immediately decided that he would make an ex¬ 
cellent subject and that I would see whether I could alter his 
decidedly prejudiced opinion of radio reception. 

Being possessed of a normal pair of ear-drums the re¬ 
semblance between the voice of a soprano singer and the blare 
of a steam whistle appeals to me as being rather remote and I 
told him so. I asked him if he would like to hear the same 
soprano voice toned down to a more natural volume. No, he 
wouldn’t. Then how about listening in on the big boxing match 
that afternoon? 

He wavered and fell, although protesting loudly that he 
expected it would be terrible and that he didn’t know why he 
should waste his time. Still, the ringside seats were $15.00 and 
he at least wanted to know the result of the fight. 

He accompanied me to my studio where I actually have a 
radio receiver installed for no other reason than to enjoy the 

broadcasting. . . 

For a few minutes we listened to some pianoforte selections. 
A strange expression of puzzled wonderment had come over my 
companion’s countenance. 

“Is that radio?” he asked. 

VII 


I assured him that it was. 

“Why, it’s marvelous,” he declared. “I never heard anything 
like it. I could swear there was somebody playing a piano in 
the next room.” 

His swift reversal of opinion concerning radio was the 
most convincing proof I have ever witnessed that over-amplifica¬ 
tion is more of a curse than a blessing. 

Making ourselves comfortable, we tuned in to the station 
which was broadcasting the prize-fight. The preliminaries were 
over and the big fight was about to begin. The clanging of 
the bell was heard calling for silence as Joe Humphreys, in his 
shrill, inimitable voice, announced the combatants. Then the 
fight was on. 

“Clang” went the bell opening the first round. 

“The men are sparring in the center of the ring,” announced 
Major White and continued to report the progress of the fight 
in his easy, convincing manner. 

By radio we were transplanted to expensive ringside seats 
and enjoyed all the thrills of watching a fistic battle. None of 
the excitement was lacking. Each blow was reported as it 
hit and we could almost hear the thuds of the blows themselves. 
The roaring of the crowd and the flying epithets of encourage¬ 
ment or anathema seemed to be a natural accompaniment of the 
scene. We were not reading about it in cold print. We were 
actually there. We couldn’t glance at the headlines and know 
beforehand the name of the winner. We had to wait in suspense 
with the rest of the crowd who were watching the fight. 

When the bell rang at the end of the fifth round Mr. Con¬ 
over removed his coat. He seemed to be laboring under the 
stress of considerable excitement and had apparently forgotten 
my existence. So far as he was concerned he was at the ring¬ 
side. 

Near the end of the next round the crowd roared with ex¬ 
citement as one of the combatants fell to the canvas. 

“One—two—three—four”—shouted the referee. 

Would he be counted out? Conover bent forward to catch 
every sound with the perspiration standing out qn his forehead. 
“Five—six—seven—clang!” 

“The bell saved him,” Major White announced. 

“Whew! That was a close shave,” said Conover, leaning 
back to wipe his forehead. “Afraid he won’t last the next round, 
though.” 

The next round ended the fight. 

For a few moments Conover sat with a dazed expression on 
his face. Then, of course, he wanted to know all about it. 

It seems unnecessary to tell the remainder of the story. Of 
course, he became an enthusiastic supporter of radio. The last 
time I heard of him he was engaged in the construction of his 
eleventh receiver!' 

Is it any wonder that radio is so popular? There is nothing 
which can take its place. There is a reality about it, a natural 
faithfulness of reproduction which cannot be equalled. To read 
about a news event after it has happened seems 1 flat and stale 

VIII 


if one can, by radio, follow its progress while it is happening. 
Music can be reproduced by radio with a perfection which is 
almost super-natural. 

I say that all these things can be. That does not neces¬ 
sarily mean that they are in every case. The horrible shrieks 
which emanate from some loudspeaking horns might be called 
super-natural but they are unpleasantly so. 

With the proper receiving set and the right kind of a repro¬ 
ducer—whether it be headphones' or horn—music and speech 
can be reproduced by radio with an unparalleled impeccability. 
The remark that “radio is about as good as a phonograph” is 
the least descriptive characterization of radio reproduction that 


it is possible to make. 

As good as a phonograph? To my mind there is as much 
difference between a phonograph and a radio receiver as there 
is between a broom and a vacuum cleaner. Both serve a useful 
purpose. You can do things with a broom that you can’t do 
with a vacuum cleaner but that doesn’t mean that the vacuum 
cleaner is outclassed by the broom. A phonograph and a radio 
receiver simply don’t compete with each other. 

But, as I have said, one must have the proper receiving 
set and the right reproducer. That is only natural. The tinny 
noises that were supposed to represent music as produced by the 
old-fashioned phonographs sounded just as bad as a poor radio 
receiver and reproducer sound today. 

One object of this book, then, is to show the construction 
of receivers' which, with the right reproducer, faithfully dupli¬ 
cate the music and voice broadcasted by radio. We are 
complete details of the design, the apparatus employed and the 
wiring diagrams of commercial receivers which accomplish this. 
These receivers, by a natural process of evolution, have been 
developed gradually. Over a period of months different de¬ 
signs and apparatus have been tried and in some cases incorpor¬ 
ated in the sets. If a change was made, it improved the sensi¬ 
tiveness, the selectivity, the durability, the appearance or the 
ease of operation. Each and every receiver was tested twice 
before it was permitted to leave the factory. As hundreds ot 
these receivers have passed through our hands it will be ap¬ 
preciated that we have had ample opportunity to locate any 
faulty design or apparatus. If the same fault made its appear- 
ance too often in testing, the source of trouble was removed 
and a new type of apparatus took its place in all subsequent sets. 

Usual or unusual faults were rectified until the apparatus 
and design of the receivers were brought to such a state of per¬ 
fection that the testing of each one is now only a matter of a few 
moments. Rarely is any trouble encountered. 

It is these finished products that we are now revealing for 
the benefit of the amateur who wants to make his own set. 1 he 
instructions are explicit so that even without a knowledge of 
the principles of radio reception any one can make one of ithese 
receivers P If you are not interested in the theory of radio re¬ 
ception, by all means turn to Part 2 and learn how hyper-sensi- 
tive receivers are made. 


IX 


For those who desire to know more about the principles 
involved, Part i outlines the theories of radio reception, with 
particular reference to radio frequency amplification. Of course, 
the receivers described in this book will employ radio frequency 
amplification. I recently received a letter from a gentleman in 
Cuba informing me that he had heard a California broadcasting 
station using only a small loop as antenna. He has the set 
which is described in Lesson io of Part 2 . If a man in Cuba 
can receive a broadcasting station in California with a small 
loop as antenna—even though it be a freak occurrence which 
might not happen again in years—there seems to be little ques¬ 
tion that radio frequency amplification made it possible. It also 
seems to demonstrate that, if it is inconvenient for the city 
dweller to erect an outdoor aerial, a loop antenna, with radio 
frequency amplification, will admirably serve the purpose. 

There are many other advantages which radio frequency 
amplification offers. This book tells about them. 

Kenneth Harkness. 

New York, N. Y., June, 1923 . 


X 


PREFACE TO THE SECOND EDITION 

When the first edition of this book was published a few months 
ago the use of Radio Frequency Amplification was still limited. It 
was considered by many to be impractical as a means of aiding short 
wave reception. This pernicious and all too persistent fallacy has at 
last been successfully confounded and today Radio Frequency Ampli¬ 
fication is the vogue. In one form or other it has been adopted by 
both radio manufacturers and amateur radio constructors. True, 
some of the systems in use are comparatively inefficient, but the 
trend of progress is in the right direction. 

During the winter of 1923 the so-called “Neutrodyne” sys¬ 
tem has had la reign of great popularity and this has fortunately 
paved the way to further improvements in the tuned Radio Fre¬ 
quency Amplifying system of reception. Our own “Harkness Re¬ 
ceiver, v employing tuned Radio Frequency Amplication, has entered 
the ranks of Renown and daily becomes more prominent. The as¬ 
cension of this circuit to its present position of eminence has not 
been brought about by means of advertising. The merits of the 
system are alone responsible for its popularity. The circuit was 
initially made public in the first edition of this volume. Then 
“Radio Broadcast,” a popular radio (magazine with a national cir¬ 
culation, recognized the numerous advantages of the receiver and 
published an article by the present Author in the November, 1923, 
issue. The article aroused so much interest that each succeeding 
issue of the magazine contained more information on the subject, 
and to satisfy the great number of readers who were unable to 
procure the November issue the Editor actually found it desirable 
to reprint the original article in the April, 1924, issue. Other radio 
magazines and newspapers also featured the Harkness Receiver in 
their publications. Amateur constructors, ever on the qui vive for 
something new, built receivers using the circuit and were pleas¬ 
antly surprised to find that the system did more than was claimed 
for it. They found that, by using this circuit with only one vacuum 
tube, they were able to produce signals of sufficient strength to oper¬ 
ate a loudspeaker with ease and receive distant stations within a 
radius of 1,000 to 2,500 miles. Since the system has only two con¬ 
trols they found the receiver unusually easy to operate; yet the tuned 
circuits insured high selectivity. Most pleasing of all was the dis¬ 
covery that the receiver actually does not oscillate, and that they 
could tune in stations without generating a single whistle or squeal. 

XI 


The good tidings of this latest and most useful type of receiver 
was spread abroad by those who had constructed it and who were 
enthusiastic and unstinting in their praise of its merits. As a re¬ 
sult, thousands of “Harkness Receivers” have been built and the 
entire output of a large factory was required to supply the demand 
for parts before a single line of advertising was inserted to promote 
their sale. 

It is important to realize that the extended use of the “Hark¬ 
ness Receiver” will greatly improve the quality of radio reception 
in general by reducing the interference caused by oscillating regen¬ 
erative receivers. The latter are directly responsible for the whistles 
and squeals which now interfere with radio reception, whereas the 
“Harkness Receiver” does not re-radiate and cannot interfere with 
the reception of others. If regenerative receivers for broadcast re¬ 
ception become extinct—as we hope they eventually will—owners 
of radio receivers will be able to listen in to the radio broadcast 
without hearing a single whistle or squeal. Distant stations will be 
received with far greater ease than at present and without the chirp¬ 
ing, twittering chorus which now accompanies DX reception. 

If you, a reader of this book, possess a regenerative receiver 
you must face the fact that you are causing interference. When 
you tune in your receiver tonight realize that, with each turn of 
your dial, you are causing shrill whistles and discordant wails to 
emanate from a thousand loudspeakers in the homes of your neigh¬ 
bors near and far. 

The Harkness Receiver does not oscillate and cannot cause in¬ 
terference to others. Yet you can receive farther and produce in¬ 
finitely louder signals with a non-interfering “Harkness Receiver” 
than you can with an interference-producing regenerative receiver. 
Moreover, the operation is simpler, the selectivity higher, the repro¬ 
duction clearer and the cost is lower. Therefore if you discard 
your regenerative receiver and build a “Harkness Receiver” to take 
its place your neighbors will not only be relieved from a great deal 
of unnecessary interference, but you yourself will receive far bet¬ 
ter than before and with less expense. 

For the second edition of this volume we have entirely re¬ 
written and greatly enlarged the chapter which describes the “Hark¬ 
ness Receiver/’ and have given complete instructions for building 
both the single-tube and two-tube models. The very latest and most 
improved type of these receivers are illustrated and explained. 

The Author wishes to thank the many readers of the first edi¬ 
tion who wrote him expressing their appreciation of his work and 

XII 


those who have written him reporting the highly satisfactory opera¬ 
tion of their home-built “Harkness Receivers.” He has endeav¬ 
ored to answer these letters individually, but if, at times, other 
duties have intervened to prevent this, he feels confident that those 
who failed to receive a personal response will accept this expression 
of thanks. 

Kenneth Harkness. 

New York, February, 1924. 


XIII 




























i • 

































































































































































' 






































































CONTENTS 


PART ONE 

THEORY OF RADIO RECEPTION 

PAGE 

Lesson 1 . ELEMENTARY LAWS OE ELECTRICITY 1 
Matter and Electricity—The Electron—Electric 
Strains—Electrical Pressure—Potential Difference 
—Capacity—Electromotive Force—Measurement of 
Current—Resistance—Ohm’s Law—Direct Current 
—Alternating Current — Frequency — Current in 
A.C. circuit with resistance only. 

Lesson 2 . GENERAL OUTLINE OF RADIO 

COMMUNICATION . 7 

Principles of Radio Communication—Measurements 
of Waves—Amplitude—Wave groups—Undamped 
Waves—Damped Waves—Modulated Waves—Ve¬ 
locity—Wavelength—Frequency—Form and Fre¬ 
quency of E.M.F. induced by Radio Waves—Differ¬ 
ent types of Radio Telegraph Transmission—Radio 
Telephony—Magnitude of E.M.F. in Receiving An¬ 
tenna—Principles of Detection—Detection of Un¬ 
damped Waves. 

Lesson 3 . INDUCTANCE—CAPACITY- 

RESONANCE . 23 

Electro-Magnetism—Law of Induced E.M.F.—Self- 
Induction—Direction of Self-Induced E.M.F.— 

Value of Self-Induced E.M.F.—Unit of Inductance 
—Effects of Inductance in a D.C. circuit—Current in 
A.C. circuit with Inductance and Resistance—Induc¬ 
tance Reactance—Factors governing Capacity— 
Mechanical Analogy of Capacity—Measurement of 
Capacity—Discharge of a Condenser—Current 
Flow in A.C. circuit with Capacity only—Capacity 
Reactance—Resonance—Oscillatory Circuits—Tun- 
ing an Oscillatory Circuit—The Resonance Curve— 

Effect of Resistance on Resonance Curve—Damping 
—Resistance of an Oscillatory Circuit—Conductor 
Resistance—Resistance of an Iron Core Coil— 
Radiation Resistance—A simple Receiving Circuit. 

Lesson 4 . CURRENTS IN COUPLED CIRCUITS ... 44 

Different Kinds of Coupling—Mutual Induction— 

Value of E.M.F. Induced by Mutual Induction— 
Coefficient of Coupling—Direction of Induced 
E.M.F.—Effect of Coupling upon Resonance Curve 
—Transformers. 




CONTENTS—Continued 


PAGE 

Lesson 5. THE VACUUM TUBE DETECTOR. 55 

Theory of Operation—Filament Circuit—Current in 
Plate Circuit—Value of Plate Current—Unilateral 
Conductivity of Tube—The Grid—Effect of Grid 
Potential on Plate Current—Receiving Circuit with 
V.T. Detector—The Operating Point—Methods of 
Adjusting Operating Point—The Triode as Detec¬ 
tor with Grid Condenser. 

Lesson 6. APPLICATIONS OF THE FEED¬ 
BACK PRINCIPLE. 66 

Feed-back System with Inductive Coupling—Self- 
generation—Regeneration—Autodyne Reception of 
Undamped Waves—Preferred System for Detecting 
Undamped Waves—Feed-back system with Capaci¬ 
tive Coupling—Single-circuit v. Loosed-coupled Re¬ 
generative Receivers. 

Lesson 7. RADIO AND AUDIO FREQUENCY 

AMPLIFIERS . 74 

The Triode as an Amplifier—Functions of Radio 
and Audio Frequency Amplifiers—Classification of 
Amplifiers—Resistance-Coupled Amplifiers—Volt- 
age Amplification of Resistance Amplifier—Repeat¬ 
ing Action—Grid Condenser and Leak—Resistance 
Amplifier for Audio Frequencies—Resistance 
Amplifier for Radio Frequencies—Inductance-Coup¬ 
led Amplifiers—For Audio Frequencies—For Ra¬ 
dio Frequencies—Transformer-Coupled Amplifi¬ 
ers—For Audio Frequencies—The Audio Frequency 
Transformer—For Radio Frequencies—Tuned Ra¬ 
dio Frequency Amplifier—Modified Tuned R.F. 
Amplifier—The Lowell R.F. Transformer-Coup¬ 
led Amplifier—Voltage Amplification of DX Trans¬ 
former-Coupled Amplifier—Controlling Self-Oscil¬ 
lation in a Radio Frequency Amplifier—By Nega¬ 
tive Feed-back—Neutrodyne System—Potentio¬ 
meter Method—The Reflex System. 

Lesson 8. AUDIBILITY AND SELECTIVITY OF 

RECEIVING SYSTEMS . 100 

Definition of Audibility—Definition of Selectivity— 
Audibility vs. Selectivity—Receiving Systems 
Compared—Single Circuit Non-Regenerative Re¬ 
ceiver—Inductively-Coupled Non-Regenerative Re¬ 
ceive r—Inductively-Coupled Regenerative Re¬ 
ceiver—Semi-tuned Transformer-Coupled R.F. Am¬ 
plifying Receiver—The Harkness Coupler—Tuned 
Transformer-Coupled R.F. Amplifying Receiver— 
Principles of Loop Reception—Efficiency of Loop 
Reception—Combining Loop and Aerial Reception. 

XVI 






CONTENTS—Continued 

PART TWO 

CONSTRUCTION OF RECEIVERS 

PAGE 

Lesson 9. RADIO-AUDIO FREQUENCY 

AMPLIFYING UNITS. 115 

Lesson 10. RECEIVER TYPE R.G. 510. 127 

Complete Aerial or Loop Receiver with Tuner, 2-stage 
Radio Amplifier, Detector and 3-stage Audio Amplifier. 

Lesson ii. RECEIVER TYPE R.G. 500.. 137 

Tuner and 2-stage Radio Frequency Amplifier for In¬ 
creasing the Range of Any Receiver. 

Lesson 12. THE “NEUTRODYNE” SYSTEM. 148 

Detecting C.W. Signals with a Radio Frequency Am¬ 
plifying Receiver. 

Lesson 13. THE “HARKNESS” RECEIVER. 156 

Complete Instructions for Building the 1 and 2-Tube 
Models of This Popular Receiver. 

Lesson 14. TROUBLE-SHOOTING HINTS. 176 

Care of Receivers—Erecting an Aerial. 


XVII 











































PART 1 


THEORY OF RADIO RECEPTION 















LESSON 1. 


ELEMENTARY LAWS OF ELECTRICITY. 


1 . To understand the principles of radio reception one must 
be fairly well grounded in the laws of elementary electricity. 
These are outlined in this chapter. Of necessity our definitions 
must be brief. To those who desire further explanation we 
recommend the perusal of an elementary text-book on electricity. 

2. Matter and Electricity. Under certain conditions matter 
is known to us as solids, liquids and gases. It will be remem¬ 
bered, however, that matter is classified under the headings of 
elements and compounds. An element is a substance which 
cannot be decomposed or chemically changed. A compound is 
a combination of elements. The molecule is the smallest par¬ 
ticle of a compound while the atom is the smallest portion of 
an element which can combine with the atoms of other elements 
to form a molecule. Thus one molecule of water is composed 
of two atoms of the element hydrogen and one atom of the ele¬ 
ment oxygen. Later discoveries of science disproved the theory 
that the atom was indivisible. Although the atom is so small 
that it cannot be seen with the most powerful microscopes, its 
actions have been studied. Just as we know of the presence 
of air by the force and energy it displays, so the movements of 
atoms have been studied and investigation has made it clear 
that each atom contains still smaller things which are known 
as electrons. Some scientists have even succeeded in establish¬ 
ing the weight and size of electrons and the speed at which they 
travel. 

3. The Electron is the unit of electrical energy. It repre¬ 
sents a charge of negative electricity. Each atom of matter 
normally possesses a certain number of electrons. Electrons may 
be removed from atoms by the application of heat or electrical 
pressure. 

4. Electric Strains: When the atoms of a body lack their 
full complement of electrons, the body is said to be positively 
charged with electricity. When the atoms of a body have more 
than their normal supply of electrons, the body is said to be 
negatively charged with electricity. 

5. When a body is thus robbed or over-supplied with elec¬ 
trons, it is not in a normal state and it has been found that 
it exerts a force or strain in the space surrounding it. This 


2 


THEORY OF RADIO RECEPTION 


strain may be represented in an illustration by lines although, of 
course, the force is invisible. Thus Fig. 1 shows lines of electric 
strain emanating from a body which is positively charged, or 
which lacks electrons. In a similar way the negatively charged 
body exerts a strain but in this case the direction 
of the lines of electric strain is inwards, towards 
the body itself. 

6 . The presence of these strains exerted by 
positively or negatively charged bodies can 
clearly be demonstrated by simple experiments. 
These experiments establish some fundamental electrical laws. 
If two positively charged bodies are brought close together, 
the strains are exerted by the two bodies oppose each other. 
If the bodies are composed of very light material the effect 
of this opposition is plainly evident. The two bodies are 
forced apart. On the other hand, if one body is positively 
charged and the other is negatively charged, the electric strains 
surrounding each body causes them to be attracted to one an¬ 
other. Like charges of electricity repel and unlike charges at¬ 
tract. 

7 . Electrical Pressure: If two 

bodies are charged with unequal 
shares of electricity and are then con¬ 
nected together by a copper wire, as 
in Fig. 2 , electrons flow from one to 
the other to equalize the share of electrons. This does not 
necessarily mean that one body has a greater quantity of elec¬ 
tricity than the other. For instance, a small body may have a 
greater share of electricity than a large body, although the 
former may have a smaller quantity than the latter. This will 
be explained further in a moment. 

8 . The excess electrons in the body B of Fig. 2 flow through 
the wire into A as soon as the path is provided. Before that 
time the space between the two bodies is strained. There is 
a certain effort on the part of the electrons in B to reach A. 
This effort is known as electrical pressure. 

9 . Potential Difference: A difference of potential is said to 
exist between two bodies which do not possess an equal share 
of electrons. If they are provided with a conducting path elec¬ 
trons will flow from one to the other to equalize the share. 
When we speak of potential or difference of potential, we must 
therefore have two points in mind. 

10 . The earth is such a large body that its electrical condi¬ 
tion is constant. Any excess or loss of electrons is immediately 
equalized. The earth, therefore, has zero potential. 

11 . Electrical pressure is measured in units called Volts. 
If any object by itself is said to have a positive potential of 20 
volts it means that the difference in potential between the object 
and the earth is 20 volts. The object, being positively charged, 
lacks electrons. The force exerted by the electrons of the earth 


Me*-? 

+ +J-~flrcfc>r7 /7<w —\^~ ~J 
3 3 

Fig. 2 



Fig. 1 



LAWS OF ELECTRICITY 


3 


to reach the positively charged body and reduce its potential 
to zero is measured as 20 volts. But if it is said that there is 
a difference of potential of 20 volts between two objects, this 
does not necessarily mean that one object is at zero potential and 
the other at 20 volts positive or negative. If one object has 
a positive potential, with respect to the earth, of 60 volts and 
the other a positive potential, with respect to the earth, of 80 
volts, the difference of potential between the two objects is 20 
volts and it is this difference which forces a flow of electrons 
from one to the other if a conducting path is provided. 

12. Capacity: As we intimated in Paragraph 7, the potential 
to which an object is raised by a charge of electricity does not 
solely depend upon the quantity of electricity, with which the 
object is charged. It also depends upon the size of the object 
and upon other considerations which we shall investigate later. 
These govern what is called the “Capacity” of any conductor. 
Capacity describes the ability of a body to hold a charge of elec¬ 
tricity. A given charge will raise the potential of. an object 
with a small capacity to a higher value than one with a large 
capacity. 

13. Electromotive Force: The flow of electrons between the 
bodies B and A of Fig. 2 is practically instantaneous and as soon 
as the potential difference between the two is reduced to ^ero, 
no further flow of electrons takes place. 


flow of f/acfcons 


Now if, while these two bodies are joined together by a 
wire, the potential difference between them can be maintained, 
there will be a continual flow of electrons from B to A and a 
continuous current of electricity will pass through the copper 
wire. To maintain this potential difference.it will evidently be 
necessary to constantly supply the body B with electrons to keep 
it negatively charged and constantly remove electrons from A 
to keep it positively charged. 

14. There are different 
methods which can be used to 
maintain potential difference 
between the two ends of a con¬ 
ductor through which a cur¬ 
rent continually flows. The 
simplest way is by chemical 
action. 

15. In Fig. 3, the plate A 
is of copper and the plate B of 
zinc. When these are im¬ 
mersed in acid, chemical action takes place which causes elec¬ 
trons to leave the copper plate and enter the zinc plate. The 
zinc gains more than its normal supply of electrons and is con¬ 
sequently charged negatively while the copper loses some of its 
electrons and is positively charged. The two plates are there¬ 
fore in the same electrical condition as the bodies A and B of 



Fig. 2. , . 

If the two plates are joined together by a copper wire, elec¬ 
trons flow from B to A, but the chemical action continues to 













4 


THEORY OF RADIO RECEPTION 


remove electrons from the copper plate and add electrons to the 
zinc plate so that the potential difference between the two plates 
is maintained and a continuous current of electricity flows 
through the copper wire. 

16. Any apparatus which, by chemical action or otherwise, 
is able to maintain one of its terminals at a higher potential than 
the other, even while current is flowing through it, is said to 
develop electromotive force. This expression is usually con¬ 
tracted to e.m.f. 

17. The arrangement illustrated in Fig. 3 is called a pri¬ 
mary cell. Cells are connected together in series to form a “bat¬ 
tery.” The object is to increase the available potential difference. 

Thus, the difference in poten¬ 
tial between the terminals of 
a battery of three cells is three 
times as great as the potential 
difference of each cell, if they 
are all equal. There are many 
different kinds of batteries but 
they serve the same purpose. 

Fig. 4 They are all sources of e. m. f. 

Fig. 4 shows two of the batteries used in radio reception, with 
the symbol employed to represent a battery in a wiring diagram. 

18. Measurement of Current: Electrons in motion constitute 
an electric current. Electrons pass through the copper wire of 
Fig. 2. Electrons flow from the zinc to the copper plate of Fig. 
3, through an outside wire. In each case an electric current is 
said to flow. The magnitude of a current is measured by the 
quantity or number of electrons' which pass a given point in 
one second. The Ampere is the unit of current. If a certain 
number of electrons pass a given point each second, the current 
is said to be one ampere. 

19. Direction of Current: In Figs. 3 and 4, we have shown 
the current as flowing from a point of negative potential to 
a point of positive potential. This is in accordance with the 
now accepted electron theory. However, years ago, before much 
was known about electricity, if was arbitrarily decided that elec¬ 
tric current flows from positive to negative. This early con¬ 
ception has not been corrected. Current is still said to flow 
from a point of positive potential to one of negative potential. 
It should be realized, however, that the movement of electrons, 
which actually constitutes an electric current, is invariably from 
negative to positive. 

20. Resistance: All substances offer resistance to the passage 
of an electric current. Resistance in electricity corresponds with 
friction in mechanics. In both cases energy is consumed in 
the production of heat. Some substances offer such a high re¬ 
sistance to the passage of electricity that they are called non¬ 
conductors or insulators. Other substances, particularly metals, 
offer low resistance and are known as conductors. 

The resistance of a conductor depends upon the material of 
which it is composed, its size and its length. Different metals 







LAWS OF ELECTRICITY 


5 


have different values of resistance. A long wire offers more re¬ 
sistance than a short one. Similarly a wire of small diameter 
offers more resistance than one of larger diameter. 

21. The Ohm is the electrical unit of resistance. A con¬ 
ductor with a resistance of 1 ohm requires an e.m.f. of 1 volt 
to force a current of 1 ampere through it. 

22. Ohm’s Law: It is evident from the foregoing that there 
is a relationship between e.m.f., current and resistance. We can 
increase the current passing through a given conductor by in¬ 
creasing the voltage of the e.m.f. acting on the ends of the con¬ 
ductor. We can also increase the current passing through a 
conductor across which a given e.m.f. is applied by decreasing 
the resistance of the conductor. 

23. The relationship between e.m.f., current and resistance 
is stated by Ohm’s Law, as follows: 

The current flowing through a circuit is directly proportional 
to the e.m.f. across it and inversely proportional to the resistance 
of the circuit. 

Volts 

Or Amperes= - 

Ohms 

24. Circuit: The term "circuit” is used to describe the path 
of an electric current. Continuous current flows from one side 
of a source of e.m.f. through a conductor or conductors and back 
again through the source of e.m.f. This circular movement of 
the current continues as long as the source of e.m.f. is maintained 
and as long as the continuous path or circuit remains unbroken. 
In this circuit may be connected apparatus through which the 
current passes to provide light, heat or to serve some other use¬ 
ful purpose. The apparatus in the circuit must, of course, pro¬ 
vide a conducting path for the current or the circuit will be 
broken and no current can flow. 

25. Direct Current: The current which flows through a cir¬ 
cuit connected across a battery is called a Continuous or Direct 
Current (D. C.) because it continually flows in one direction. 

26. Alternating Current: The current which flows in a cir¬ 
cuit connected across an alternator, which generates an alternat¬ 
ing e.m.f., continually reverses in direction and is called an Al¬ 
ternating Current (A. C.). 

27. The e.m.f. generated by an alternator can best be under¬ 
stood by studying the curve of Fig. 5. The comparison of values 
which are made possible by a curve of this type should be fully 
appreciated. 

28. In this curve diagram the elapse of time is measured 
in fractions of seconds along the horizontal axis from the point 
of zero. The values of the factor or factors which are to be com¬ 
pared as time elapses are measured along the vertical line from 
the point of zero. 

At this time we wish to compare the different values and di¬ 
rection of an alternating e.m.f. while time elapses. An alternat¬ 
ing e.m.f. does not always act in the same direction. It is con¬ 
tinually reversing in direction. At one moment one of the ter- 



6 THEORY OF RADIO RECEPTION 

minals of the alternator is positive with respect to the other and 
the next moment it is negative. 

29. We can show the direction as well as the value of the 
e.m.f. at any moment of time by measuring the values of the 

e. m. f. in a positive direction 
along the vertical line above 
the point of zero and by 
measuring the values of the 
e. m. f. in the opposite or nega¬ 
tive direction along, the verti¬ 
cal line below the point of zero. 

In this way the’ curve of 
Fig. 5 shows clearly the values 
and direction of the e. m. f. as time elapses. 

30. It will be noted from this curve that the voltage gen¬ 
erated by an alternator passes through a definite cycle of values 
and direction. Commencing at zero the voltage increases rapidly 
and then less rapidly until it reaches a maximum in a positive 
direction. Then it decreases, slowly at first but more rapidly 
later until it falls to zero. At this moment it reverses direction, 
increases to a maximum in a negative direction and again falls 
to zero. This complete cycle is continually repeated. 

31. The Frequency of an alternating e.m.f. is represented by 
the number of complete cycles of voltage generated per second 
of time. The frequency of the alternators commonly used for 
lighting circuits is about 60 cycles per second. 

32. Current in A. C. circuit with Resistance Only: The cur¬ 
rent which flows in a circuit across which an alternating e.m.f. 
is applied must, of course, be an alternating current. We will 
see later that there are certain factors in a circuit which affect 
the value of an alternating current. If we consider, however, 
that a circuit only possesses resistance, the strength of an al¬ 
ternating current at any instant of time can be determined by 
Ohm’s Law. 

33. In Fig. 6, the cur¬ 
rent curve is shown with 
the voltage curve. The 
conditions represented by 
these curves' obtain when 
the circuit has resistance 
only. The value of the cur¬ 
rent at any moment can be found by dividing the voltage at that 
instant by the resistance of the circuit, in accordance with 
Ohm’s Law. Consequently, the current is in phase with the 
voltage and has a similar form. 












LESSON 2. 


GENERAL OUTLINE OF RADIO COMMUNICATION. 

34. Radio magazines and newspapers are frequently asked 
to answer the question, “What is the difference between radio 
and audio frequency amplification ?” The questioner probably 
has a radio receiver with three vacuum tubes. The first tube 
is the “detector” and the other two the “amplifiers.” There 
seems to be no logical place in the receiver for the addition of 
further amplification. The signals are loud. If a third stage of 
amplification were added, the nearby stations might be unpleas¬ 
antly loud. 

35. But if this typical radio receiver is analyzed for a mo¬ 
ment it will be realized that the amplifying tubes and trans¬ 
formers are merely magnifying a signal which is made audible 
by the detector tube itself. The function of the detector tube 
and its circuits is to convert the energy induced in the receiv¬ 
ing antenna by passing radio waves into currents which will pro¬ 
duce audible sounds when they pass through a telephone re¬ 
ceiver. The manner in which the tube and its circuits accomplish 
this is naturally somewhat complicated but it is probably known 
to all that the amplifying tubes take no part in this operation. 

36. The variations of current in the output circuit of a 
detector tube will cause a telephone receiver diaphragm to. vi¬ 
brate if the telephone is included in this circuit. If the telephone 
is held to the ear, the vibrations of the diaphragm will be “heard.” 

37. These variations in the output circuit of a detector tube 
may be impressed upon the input circuit of an amplifying tube 
through a transformer. The variations will be greatly magnified. 
If the telephone is included in the output circuit of the amplify¬ 
ing tube, the vibrations of the diaphragm will be much more 
pronounced and a louder sound produced. The amplified varia¬ 
tions can be still further magnified by impressing them upon the 
input circuit of a third tube. Usually two or three stages of such 
amplification are sufficient for all practical purposes. 

38. This type of amplification in a radio receiver is there¬ 
fore called “audio frequency amplification,” and refers to the 
magnification of the audio frequency current variations in the 
output circuit of the detector tube. 

39. It will be evident, however, that audio frequency am¬ 
plification does not actually increase the sensitiveness of a radio 


8 


THEORY OF RADIO RECEPTION 


receiver. It only magnifies signals which are made audible by 
the action of the detector tube and its circuits. If the waves 
radiated by a distant transmitting station do not induce sufficient 
energy in the input circuit of the detector tube to operate this 
device, the signals will not be audible in the output circuit. Nor 
will they be audible in the output circuits of the first, second 
or third audio frequency amplifying tubes. These tubes and 
their circuits merely amplify the audio frequency variations in 
the output circuit of the detector tube. 

40. The amplifying properties of the vacuum tube, how¬ 
ever, can be utilized in another manner to increase the sensi¬ 
tiveness of a radio receiver. The current which is induced in 
the receiving antenna by radio waves may be amplified before 
it is impressed on the input circuit of the detector tube and con¬ 
verted into sound. This method is known as “radio frequency 
amplification.” 

Without this amplification the currents induced in the re¬ 
ceiving antenna by waves radiated from distant transmitting 
stations may frequently be too feeble to operate the detector and 
no signal can be heard. Radio frequency amplification magnifies 
these currents before they are impressed upon the detector cir¬ 
cuit and the signal becomes audible. 

41. It may be said, then, that radio frequency amplification 
increases the range of any radio receiver. For instance, if, under 
freak conditions, an amateur is able to receive signals transmitted 
by a station a thousand miles distant, the addition of two stages 
of radio frequency amplification will ensure the nightly reception 
of this station and probably many stations much more distant 
which he could not possibly hear without radio frequency am¬ 
plification. 

42. Principles of Radio Communication: To understand the 
nature of the currents which are induced in a receiving antenna 
by radio waves and which may be magnified by radio frequency 

amplification, it is necessary to 
have a general conception of 
the principles' of raido com¬ 
munication. 

43. The transmission and 
reception of radio wa^ } may 
be compared with the produc¬ 
tion and reception of sound 
waves. As an illustration of the 
manner in which sound or 
pressure waves are produced 
and radiated, Fig. 7 depicts the 
radiation of waves set up by 
the vibrations of a wire which 
has been struck by a hammer, as in a piano. This sound 
wave depends for its existence upon the variations of air pressure 
above and below normal pressure produced by the vibrations 
of the wire. 

















































OUTLINE OF RADIO COMMUNICATION 


9 


44. The vibrations of the wire may be represented by the 
curve of Fig. 8. Distances along the vertical line in either direc¬ 
tion from the point of zero represent, in this case, the movement 
of the wire from its normal position. All points along the ver¬ 
tical line above zero represent 
distances in one direction, 
while all points on the vertical 
line below the horizontal rep¬ 
resent distances' in the opposite 
direction. It will be noticed 
from this curve that the vibra¬ 
tions gradually subside. The 
initial elastic energy furnished 

■ by striking the wire is lost in overcoming the friction of air. 

45. The effect of a single to and fro vibration on the air 
surrounding the wire is to increase its pressure above normal, 
reduce it to normal, lower it below normal and again raise it to 
normal. This complete cycle is repeated again and again as long 
as the wire vibrates. 

46. In accordance with the principles of wave motion, a 
group of waves is radiated in the form of expanding spherical 
areas of compressed and rarefied air as illustrated in Fig. 7. Each 
complete cycle of vibration produces a single wave. 

47. When these waves strike the ear-drum of a listener 
within their radius they cause it to vibrate at the same frequency 
and with the same form as the vibrations of the wire which pro¬ 
duced the waves. These vibrations of the ear-drum create the 
sensation of sound in the brain of the listener. 

48. The vibrations of the wire in this illustration may be 
taken to represent the alternating current which is caused to 
flow in the aerial of a radio transmitting station. These alterna¬ 
tions of current cause strains in the surrounding space and radio 
waves are set in motion which radiate in all directions with the 
speed of light. 

49. At the receiving station, the energy of the successive 
waves is impressed upon the receiving aerial and a feeble alter¬ 
nating e.m.f. is induced having the same frequency as the al¬ 
ternating current in the transmitting aerial and of similar form. 
The value of the alternating current which flows in the receiving 
aerial as a result of this induced e.m.f. depends upon other fac¬ 
tors which we will consider later. 

MEASUREMENTS OF WAVES. 

50. In Fig. 9, representing a group of waves, we have indi¬ 
cated some of the terms which are used to describe the meas¬ 
urements of waves. We will consider the meaning of these and 
other terms. 

51. The amplitude of any wave is the measurement of the 
maximum energy contained in the wave. This is represented in 
the curve by the distance from the normal level to the highest 
point. In the case of the familiar water wave it is evident that 






10 


THEORY OF RADIO RECEPTION 



a high wave contains more energy than a low one. The high 
wave is said to have a greater amplitude than the low wave. 
The amplitude or measurement of maximum energy contained 
in a sound wave is represented by the degree of compression to 

which the air is raised above 
normal pressure by the 
wave. Similarly the ampli¬ 
tude or power of a radio 
wave is measured by the in¬ 
tensity of the electric field 
produced by the wave. 

52. The amplitude of a wave decreases as the wave travels. 
This can easily be observed in water waves. The ripple of waves 
which spreads out over the surface of a pond into which a pebble 
is dropped commences from the starting point with maximum 
amplitude. The waves gradually flatten out as they travel out¬ 
wards in circles over the surface of the water until the energy 
is entirely expended. 

53. The distance which radio waves travel, then, depends 
upon the initial amplitude of the waves at the transmitting sta¬ 
tion and the amount of energy in the waves at any given distance 
from the transmitter also depends upon this initial amplitude. 
The initial amplitude, of course, is governed by the value of 
the alternating current in the transmitting aerial which is pro¬ 
ducing the waves, just as the initial amplitude of sound waves and 
the distance they travel depends upon the strength of the vibra¬ 
tions which are producing them. 

54. The expression amplitude is also used to describe the 
maximum value reached by an alternating current during one 
alternation. Referring to the curve of Fig. 6 it will be noted that 
each alternation of current commences with a zero value, gradu¬ 
ally increases until it reaches a maximum and again decreases 
to zero. 

The amplitude of any particular alternation is the measure¬ 
ment of the maximum value the current attains during that al¬ 
ternation. This is represented by the vertical distance from the 
horizontal line to the highest point of the curve. 

55. The amplitude of an alternating current, therefore, is 
evidently a measurement of the energy in the circuit in which 
the current flows. If the amplitude is increased, the available 
energy is increased and vice versa. 

56. It may be said, then, that the initial amplitude of a 
radio wave depends upon the amplitude of the cycle of alternat¬ 
ing current producing the wave. 

57. Wave Groups: A single wave is produced by each com¬ 
plete cycle of alternating current in the transmitting aerial. If 
the alternating current continues to flow, a group of waves is 
radiated. The number of waves in the group, of course, is the 
same as the number of cycles of alternating current. More¬ 
over, the amplitude of each successive wave in the group de¬ 
pends upon the amplitude of each successive cycle of alternating 
current. 






OUTLINE OF RADIO COMMUNICATION 


11 


58. Undamped Waves: If the amplitude of each consecutive 
cycle of alternating current is the same as its predecessor, a 
group of waves of constant amplitude is radiated, as illustrated 
in Fig. 9. Waves of this character are known as “Undamped” 
waves. 


59. Damped Waves: But if the amplitude of each consecu¬ 
tive alternation of alternating current is less than its predecessor, 
a group of waves of decreasing 
amplitude is radiated as repre- 
sented by Fig. 10. The waves hh^es 
in a group of this nature are ' 
called “Damped” waves. 

60. Modulated Waves: The 
amplitude of the waves which 
are produced by alternations of current in a transmitting aerial, 
then, follow exactly the amplitudes of the successive cycles of 
alternating current. If the amplitudes of the alternating cur¬ 
rent increase and decrease as illustrated in Fig. 11, the ampli¬ 
tudes of the waves follow 
these variations, no matter 
how complicated they may be. 
The waves in a group of this 
character are continuous but 
their amplitudes are “modu¬ 
lated.” 

61. Velocity: A radio wave travels at a definite speed. The 
velocity of a wave or group of radio waves is 300,000,000 meters 
per second which is equal to about 186,000 miles per second. 
This almost inconceivable velocity, which is the same as that of 
light, ensures practically instantaneous communication between 
radio stations, no matter how far distant they may be from each 
other. This velocity is a constant value, irrespective of the 
amplitude or length of the wave. 

62. Wave-length: The length of a wave is the horizontal 
distance from the crest of one wave to the crest of the next. 
This measurement of radio waves is made in meters. A meter 
is 39.37 inches. The length of a wave remains constant, no mat¬ 
ter how far the wave travels. 

63. Frequency: The frequency of waves is the number of 
waves which are produced or pass a given point in one second. 
Since a single wave is produced by one cycle of alternating 
current, the number of waves which are produced in one second 
depends upon the number of cycles of alternating current in 
one second. In other words, the frequency of the waves de¬ 
pends upon the frequency of the alternating current which is 
producing the waves. If the frequency is increased, more waves 
are produced per second and vice persa. 

64. Since the velocity of radio waves is constant, we may 
establish a relationship between frequency, wave-length and 


flrvpfi/ide __ y'oleazajHf 



Fig. 11 





velocity. , f 

If a radio transmitter sends out a stream of continuous 
waves for a period of one second, the first wave will, if it is 









12 


THEORY OF RADIO RECEPTION 


powerful enough, have travelled 300,000,000 meters by the time 
the last wave is just leaving the transmitting aerial, since radio 
waves travel at the speed of 300,000,000 meters, per second. 
Therefore, the total length of this group of waves is 300,000,000 
meters. It follows that the length of each wave in this group 
must be equal to 300,000,000 meters divided by the total num¬ 
ber of waves in the group. For instance, if 15,000 waves are 
transmitted during one second, or, in other words, if the fre¬ 
quency is 15,000 cycles, the length of each wave must be 300,000,- 
000 divided by 15,000=20,000 meters. 

65. We may state this relationship in an equation as follows: 

Velocity 

Wave-length =- 

Frequency 

or Velocity=Wave-length x Frequency. 

It is evident that, since the velocity is a constant value, the length 
of a radio wave depends entirely on the frequency with which 
the waves are produced or, in other words, on the frequency of 
the alternating current in the transmitting antenna. Further, 
the higher the frequency the shorter the wave-length and vice 
versa. 

66. Now, to contain a useful amount of energy and for many 
other reasons, a radio wave cannot be much longer than 20,000 
meters'. The waves now in common use for radio communication 
measure from 150 meters to 25,000 meters. The frequency cor¬ 
responding to the wave-length of 150 meters is 2,000,000 cycles 
per second and that corresponding to 25,000 meters is 12,000 
cycles per second. The frequencies embraced by these limits 
are the frequencies of the waves now used for radio communi¬ 
cation. 

67. To produce waves of these frequencies, then, the cur¬ 
rents in the transmitting antenna must be capable of alternating 
at some frequency from 12,000 cycles per second to 2,000,000 
cycles per second, depending upon the wave-length upon which 
it is desired to transmit. The frequency of the alternating cur¬ 
rents used in lighting systems is only 60 cycles per second. Spe¬ 
cial apparatus is therefore used in radio transmission to produce 
the extremely high frequency alternating currents which are 
necessary to transmit radio waves. 

68. To distinguish high frequency alternating currents 
from those of lower frequency, the former are called “Oscilla¬ 
tions” and the currents are said to alternate at “radio frequency.” 
Hereafter, we will use these terms to describe high frequency al¬ 
ternating currents. 

FORM AND FREQUENCY OF E.M.F. INDUCED BY 
RADIO WAVES. 

69. The amplitude of a wave represents the maximum 
amount of energy contained in the wave. Therefore, a group 



OUTLINE OF RADIO COMMUNICATION 


13 


of undamped waves (of constant amplitude) as represented by 
Fig. 9 induce an undamped alternating e.m.f. in a receiving an¬ 
tenna within the radius of the waves. Although, when they 
reach the receiving station, the amplitude of each wave in the 
group is considerably less than the initial amplitude, the waves 
all decrease in the same proportion. Consequently the same 
amount of energy is induced in the receiving antenna by each 
successive wave. 

70. A group of damped waves similarly induce a damped 
alternating e.m.f. in the receiving antenna. The energy, or am¬ 
plitude, of each consecutive wave is less than its predecessor and 
consequently the e.m.f. induced in the receiving antenna by each 
successive wave is less than its predecessor. 

71. In the same way, a group of modulated continuous waves 
induce in a receiving antenna a modulated alternating e.m.f. 
The variations in amplitude bear the same relation to each other 
as the modulations in amplitude of the current in the transmit¬ 
ting aerial. 

72. It will also be evident that the frequency of the alternat¬ 
ing e.m.f. induced in the receiving antenna is the same as the 
frequency of the transmitting current. The frequency with 
which the waves are produced depends upon the frequency of 
the transmitting current. Each successive wave induces a com¬ 
plete cycle of alternating e.m.f. in the receiving antenna. There¬ 
fore, the number of complete cycles of alternating e.m.f. which 
are induced in one second (frequency) depends upon the rapid¬ 
ity with which the waves follow each other. This, in turn, de¬ 
pends upon the frequency of the transmitting current. 

It is plain, then, that the alternating e.m.f. induced in the 
receiving antenna has exactly the same frequency as the trans¬ 
mitting current. We have already noted that to produce radio 
waves the transmitting current must have a very high frequency, 
ranging from 12,000 cycles per second to 2,000,000 cycles per 
second. 

73. Therefore, the e.m.f. induced in a receiving antenna by 
radio waves is an oscillating e.m.f. and the frequency is the same 
as the frequency of the oscillations in the transmitting aerial 
radiating the waves. Moreover, the induced oscillations follow 
exactly the variations in amplitude, if any, of the transmitting 
current. 

74. Different Types of Radio Telegraph Transmission: In 

Fig. 12, we have illustrated the different types of waves which 
are radiated by transmitting stations to communicate by radio 
telegraphy. 

75. Fig. 12A represents a single train of damped waves 
radiated by a “spark” transmitter. Each of these trains is pro¬ 
duced by a damped oscillating current in the transmitting an¬ 
tenna. The trains of waves follow each other in rapid succes¬ 
sion, usually about 500 complete trains per second. 

76. Trains of damped waves are only used in communicating 
by telegraphic code, or “dot-dash” system. The telegraph key 
opens and doses the transmitting circuit. While the key is open 


14 


THEORY OF RADIO RECEPTION 

no waves are radiated. When the key is closed wave trains are 
radiated, following each other at a frequency in the neighborhood 
of 500 per second. The exact number of wave trains per second 
depends upon the “spark frequency” of the transmitter. If the 
spark frequency is 500 and the telegraph key is held down for one 

second to form the “dash” of 
some letter of the Morse code, 
500 complete trains are radi¬ 
ated during this second. The 
operator at the transmitting 
station manipulates the key in 
accordance with the spacings 
of the code. The Continental 
code is internationally used in 
radio telegraph transmission. 
At the receiving station each 
succesive train of damped 
waves induces a damped oscil¬ 
lating e. m. f. in the antenna. 
We will see later how these 
oscillations are “detected.” 

77. Fig. 12B illustrates 
the undamped continuous 
waves which are radiated by 
another type of transmitter. 
Undamped waves are pro¬ 
duced by continuous oscilla¬ 
tions of constant amplitude in 
^*2* 12 the transmitting antenna. As 

long as the telegraph key at the transmitting station is held 
down a continuous stream of undamped waves is radiated. The 
key is again manipulated in accordance with the spacings of the 
telegraphic code. 

At the receiving station each group of undamped waves, 
constituting by its length a “dot” or a “dash,” induces an un¬ 
damped oscillating e.m.f. in the antenna. Between each dot and 
dash no waves are radiated (for during these space periods the 
key at the transmitter is open) and consequently no oscillations 
are induced in the receiving antenna during these periods. 

This type of radio telegraph transmission is commonly 
called “C.W.” (continuous wave) transmission and is by far the 
most efficient method of radio communication. We will show 
later that special apparatus is necessary to receive C.W. tele¬ 
graph signals'. 

78. Another system of radio telegraph transmission is illus¬ 
trated in Fig. 12C. This is known as I.C.W., or interrupted con¬ 
tinuous wave transmission. The object of this method is to 
avoid the necessity of special apparatus at the receiving station. 
Undamped waves are radiated but they are broken up into com¬ 
paratively short groups, somewhat after the style of the “spark” 
method except that each wave train is undamped. Similar 



















OUTLINE OF RADIO COMMUNICATION 


15 


groups of undamped oscillations are induced in the receiving 
antenna. 

79. Radio Telephony: Continuous waves are also utilized 
for the transmission of radio telephony. Radio telephony may 
be defined as the reproduction at a distant point of sounds which 
are produced at the transmitting station, with radio waves act¬ 
ing as the medium or ^ - J ~ 

80. Now “sounds” 
are really pressure 
waves produced by 
vibratory motion. 

When sound waves 
strike the ear-drum 
of a listener they 
cause it to vibrate. 

The organism of 
the ear detects the 
difference between 
sound waves pos¬ 
sessing different fre¬ 
quencies and “wave¬ 
form” and thus 
translates the waves 
into musical sounds, 
speech or other 
forms of sound. For 
instance, if pressure 
waves are trans¬ 
mitted through the 
air at, say, a fre¬ 
quency of 500 cycles 
per second, the 
waves cause the ear¬ 
drum of a listener to 
vibrate at this frequency and create the sensation of a certain 
musical note. Similarly, waves of higher frequency create the 
sensation of higher notes and vice versa. But the human ear is 
only susceptible to waves below a certain frequency. If pres¬ 
sure waves are transmitted at frequencies much above 10,000 
cycles per second, the ear-drums of the average person will not 
respond. Frequencies below about 10,000 cycles per second are 
therefore called audio frequencies (or audible frequencies). The 
average frequency of the sound waves produced by the human 
voice is only about 800 cycles per second. 

81. The sound waves produced by the vibrations of the 
human voice and some musical instruments do not follow a pure 
harmonic curve. Sound vibrations are made up of numerous 
harmonics and overtones resulting in the transmission of a wave 
of varying amplitude. Thus, the “wave form” of a single vowel 
sound as pronounced by the vibrations of the voice might pro¬ 
duce a curve similar to that shown in Fig. 13A. In many cases, 


carrier or tne sounds. 


A. Sound 
wave. 


B. Micro¬ 
phone current. 


C. Modulated 
Radio waves. 



Fig. 13 
















































16 


THEORY OF RADIO RECEPTION 


the wave form is even more complicated but it invariably repeats 
itself at regular intervals. 

82. In the communication of speech or music by radio these 
complicated waves must be exactly transmitted and again repro¬ 
duced at the receiver. This would be practically impossible 
were it not for the great difference in frequency between radio 
waves and sound waves. In the early days of radio telephony, 
many experiments were made using “arc” continuous wave 
transmitters but only by the perfection of the three electrode 
vacuum tube and its use in the production of completely steady 
or undamped waves of very high frequency and also in amplify¬ 
ing audible frequency currents has it been possible to bring the 
radio telephone to its present efficiency. 

83. The continuous high frequency waves act as the “car¬ 
rier” of the low frequency sound waves. This is accomplished 
by varying the amplitude of the continuous high frequency waves 
so that the variations in amplitude conform to the frequency 
and wave-form of the sound waves. 

84. The curves of Fig. 13 show how the sound wave form 
and frequency are carried by the radio waves. 

At the transmitting station, the speaker addresses a micro¬ 
phone and curve A is supposed to represent a portion of the 
sound waves which are produced by his voice. A current of 
electricity is constantly flowing through the microphone and the 
action of the sound waves produces variations in this current; 
the current rises and falls in conformity with the frequency and 
form of the sound waves. This fluctuation of the microphone 
current is shown in Fig. 13B. The sound waves strike the micro¬ 
phone at the moment X when the speaker commences to talk. 
Before that time, the microphone current is steady. 

The radio transmitting apparatus generates and radiates a 
constant stream of radio waves of high frequency. The fluctua¬ 
tions of the microphone current, produced by the voice, are am¬ 
plified and applied to the generator of radio waves in such a way 
that the amplitude of the radio waves rises and falls at this ap¬ 
plied frequency, as indicated by Fig. 13C, thus preserving the 
wave form and frequency of the sound wave. Before the moment 
X the microphone current is steady; the amplitude of the waves 
is only varied while sound waves are striking the microphone. 

85. At the receiving station an oscillating e.m.f. is induced 
in the antenna and the amplitude of this e.m.f. is modulated in 
the same manner as the modulations of the oscillations in the 
transmitting aerial. We shall see later how the sound waves are 
reproduced at the receiving station. 

MAGNITUDE OF E.M.F. IN RECEIVING ANTENNA. 

86. The magnitude or strength of an oscillating e.m.f. in¬ 
duced in a receiving antenna depends upon various factors. 
Since the amplitude of the waves decreases as they travel, the 
value of the induced e.m.f. decreases as the distance between the 
transmitting station and the receiving station increases. Again, 


OUTLINE OF RADIO COMMUNICATION 


17 


since the initial amplitude of the waves depends upon the ampli¬ 
tude of the oscillating current in the transmitting aerial, the 
value of the oscillating e.m.f. induced in the receiving antenna 
decreases as the power of the transmitter decreases. It is con¬ 
ceivable that a higher value of e.m.f. may be induced at a receiv¬ 
ing station by the waves radiated from a high-power radio trans¬ 
mitting station 500 miles distant than the e.m.f. induced by the 
waves from a low-power station only 100 miles distant. 

87. The magnitude of the oscillating e.m.f. induced at a 
receiving station by the waves radiated by any given transmit¬ 
ting station, however, is not a constant value, even if the power 
used at the transmitter is unchanged. For instance, at night 
radio waves travel great distances without decreasing in ampli¬ 
tude to the same extent as during the day. The amplitude of 
the waves is also affected by atmospheric conditions, the nature 
of the country lying between the transmitting and receiving 
stations, etc. 

88 All the above factors, however, which govern the value 
of the oscillating e.m.f. induced in the receiving antenna, are be¬ 
yond the control of the owner of the receiving station. He can¬ 
not decrease the distance between the transmitting stations and 
his own receiver nor can he control the power used by the trans¬ 
mitters. No matter what type of receiving apparatus he em¬ 
ploys he is able to receive much better at night than during the 
day. 

89. The only controllable factor which ensures the highest 
possible value of induced e.m.f. by any given radio wave is the 
design of the receiving antenna itself. A greater value of e.m.f. 
is induced in a high outdoor aerial than a low aerial by the same 
radio waves. In either case the induced e.m.f. is a higher value 
than that induced in a low indoor antenna or a loop antenna. 
Even this factor may be beyond the control of the owner of a 
receiving station. It is sometimes inconvenient to erect an out¬ 
door aerial. The city dweller is often forced to use an indoor 
antenna. 

90. The amplitude of the current which flows in any re¬ 
ceiving antenna as a result of a given value of induced e.m.f. de¬ 
pends upon other factors which we will consider in the next 
Lesson. However, even if the antenna circuit is suitably ad¬ 
justed so that a maximum oscillating current flows as a result of 
a given value of induced e.m.f. it is conceivable that the latter 
value may be so low that insufficient energy is produced to oper¬ 
ate the “detecting” device which translates the received oscilla¬ 
tions into audible sounds. As outlined above, this may be due 
to the design of the receiving antenna itself, the distance between 
the transmitter and receiver, the power of the transmitter, etc. 

91. A Radio Frequency amplifier may be used to magnify 
these feeble oscillations induced in the receiving antenna so 
that sufficient energy is applied to the detector to operate this 
device and thereby make signals audible which would otherwise 
be too weak to operate the detector. It will now be evident that 


18 


THEORY OF RADIO RECEPTION 


this type of amplification is called “radio frequency amplifica¬ 
tion” because the oscillations induced in a receiving antenna by 
radio waves alternate at “radio frequency” and it is these high 
frequency oscillations which are magnified by the radio fre¬ 
quency amplifier. 

PRINCIPLES OF DETECTION. 


92. The necessity of some device to make the message car¬ 
ried by the radio waves perceptible to one of the human senses 
is apparent. Visual detection is sometimes utilized for radio 
telegraphy but audible detection is much more common and is, 
of course, necessary to reproduce sounds by radio telephony. A 
telephone receiver of some kind is used to convert the energy 
into air vibrations or sound. 

93. But if the telephone receivers are merely 
SU placed in series with the receiving antenna, the dia¬ 
phragms will not be vibrated by the oscillations in¬ 
duced in the antenna by radio waves, and no sound 
will be heard. The inertia of the diaphragms prevents 
them from following the extremely rapid reversals .of 
radio frequency current. In any case, even if the dia¬ 
phragms could vibrate at this high frequency, no 
sound would be heard because the frequency would be 
above the limit of audibility. 

94. It is evident, then, that the high frequency 
currents must be changed in some manner so that they 
can pass through the telephones and vibrate the dia¬ 
phragm at some audible frequency.. This can be ac¬ 
complished by means of a rectifier which assists in de¬ 
tecting the radio signals. 

14 , 95. A rectifier is a device which offers in¬ 

finite resistance to the passage of current in one direc¬ 
tion but freely allows it to pass in the opposite direction. 
This condition is never realized in practice but the more 
closely it is approached, the better is the rectification. Some 
minerals are natural recti¬ 
fiers and the vacuum tube 
can also be used to rec¬ 
tify alternating currents. 

96. If a crystal recti¬ 
fier is inserted in series 
with the receiving anten¬ 
na and telephones, as in¬ 
dicated in Fig. 14, an 
oscillating current can 
flow in the antenna in one 
direction only. 

97. Let us suppose 

that the waves from a 
spark transmitting sta¬ 
tion induce an oscillating 
voltage in the antenna. rm pi g 15 



Tefeph*le. 












OUTLINE OF RADIO COMMUNICATION 


19 


The voltage induced by two of the wave trains is graphically 
illustrated in Fig. 15A. 

The current which flows in the antenna is indicated at B 
from which it may be seen that each negative half cycle of volt¬ 
age is unable to cause a current to flow because the rectifier 
offers high resistance to current in that direction. During each 
wave-train, therefore, the radio frequency pulsations of current 
are all acting in the same direction. 


A. Sound 
Wave 



Owing to the inertia and resistance of the telephones, this 
results in a single surge of current through the telephones as 
shown at C. Each surge of current pulls the diaphragms. Since 
the wave trains follow each other at a frequency in the neighbor¬ 
hood of 500 per 
second, the dia¬ 
phragms vibrate at 
this frequency and 
produce an audible 
note. As long as 
the operator at the 
transmitting station 
holds down the tele¬ 
graph key wave 
trains are radiated 
and an audible vi¬ 
bration is sustained 
in the receiving tele¬ 
phone. Therefore, if 
the transmit ting 
operator closes the 
telegraph key and 
sends a message by 
Morse code, the sig¬ 
nals are audible in 
the receiving tele¬ 
phone and can be 
read by one skilled 
in the code. 



D. Telephone 
Current 


T/Af£ 

Fig. 16 


98. A rectifier can be used in the same way to detect sig¬ 
nals sent out by an I.C.W. transmitter, the only difference 
between the two methods of transmission being the amplitude 
and length of the wave trains. Since the trains of waves radiated 
by I.C.W. are undamped a higher value of current flows in the 
antenna during each train and consequently the telephones are 
traversed by a stronger surge of current. The audibility of the 
signals is greater. 

99 Radio telephone signals can also be detected with the 
assistance of a rectifier. To explain the manner in which speech 
and music are reproduced by the radio receiver we show in Fig. 
16A, a portion of the sound wave which is supposed to be pro¬ 
duced at the transmitting station by a speaker’s voice or a musi¬ 
cal instrument. 


















































20 


THEORY OF RADIO RECEPTION 


Fig. 16B indicates how the sound wave form and frequency 
are preserved by the modulations in amplitude of the oscillating ^ 
e.m.f. induced in the receiving antenna as previously explained. 
Unmodulated waves induce in the receiving antenna an un¬ 
damped oscillating e.m.f. but modulated waves induce an oscillat¬ 
ing e.m.f. of varying amplitude. The variations in amplitude 
have the same form and frequency as the sound wave. The high 
frequency current is rectified by the crystal. Fig. 16C illus¬ 
trates the resulting uni-directional pulses of current. 

The rectified undamped oscillations cause current to flow 
through the telephones but the value of this current does not 
change and therefore the diaphragm does not vibrate and no 
sound is heard. But the rectified oscillations of varying ampli¬ 
tude increase and decrease the value of the current passing 
through the ’phones in step with the changes in amplitude. The 
resulting telephone current is shown in Fig. 16D. The changes 
in this current have the same form and frequency as the sound 
wave produced at the transmitting station. The vibrations of 
the telephone diaphragm follow this changing current and con¬ 
sequently a sound is produced at the receiving station which 
duplicates the original sound at the transmitter. 

100. Detection of Undamped Waves. It will be noted from 
the foregoing paragraphs that a telephone receiver with simple 

rectifier can be used to de¬ 
tect every type of radio sig¬ 
nal except undamped wave 
signals. Undamped oscilla¬ 
tions are rectified but this 
does not assist in their de¬ 
tection because the continu¬ 
ous oscillations are not in¬ 
terrupted or modulated in 
any way and consequently 
the telephone current does 
not change in value except 
at the beginning and end of 
each dot and dash. The 
telephone diaphragm does 
not vibrate and no sound is 
heard. 

101. This will be made clear by the curves of Fig. )7 which 
show at A the oscillating e.m.f. induced in the receiving antenna 
by undamped waves radiated from a transmitting station. B 
shows the rectified current which flows in the circuit while C 
indicates the resulting telephone current. The latter commences 
to flow at the beginning of the wave group, maintains a steady 
value as long as the oscillations are induced and falls to zero 
when the wave group terminates. As there are no changes in 
the telephone current during this time, the diaphragm does not 
vibrate at an audible frequency. 

102. Special apparatus is therefore required at the receiving 
station to detect undamped waves. To produce an audible note 



0 

£ee/>ffec/ 

Current 


C 

7itef>hon<? 


T/M£ 

Fig. 17 










OUTLINE OF RADIO COMMUNICATION 


21 


in the 'phones, it is necessary to modulate or interrupt in some 
way the continuous oscillations induced in the antenna. This 
can best be accomplished by locally inducing in the receiving 
antenna an undamped oscillating e.m.f. which has a frequency 
slightly different from that of the incoming signal. The local 
oscillations can be generated by a small vacuum tube with suit¬ 
able circuits. This high frequency oscillator, as it is called, acts 
as a miniature transmitting station and induces an undamped 
oscillating e.m.f. in the receiving antenna in the same manner as 
the distant transmitting station. The circuits of the oscillator 
are arranged in such a way that it is possible to vary the fre¬ 
quency of the local oscillations. 

103. If the local oscillation is continuously maintained and 

no signal e.m.f. is being induced, the current flowing through the 
telephones will be as illustrated in Fig 17. A steady current 
will flow through the telephone receiver as long as the local 
oscillations are maintained. . 

104. But if, in the same receiving circuit, an oscillating e. m. f. 
of slightly different frequency is induced by the undamped waves 
of a distant transmitter, the current produced by the combined 
oscillations alternately increases and decreases in amplitude as 
the two oscillations come in and out of phase with each other. 



105. A detailed explanation of this phenomenom of “beats’' 
in the amplitude of the current produced by two super-imposed 
oscillations of different frequency is not necessary at this time. 
A study of the curves of Fig. 18 will make it clear. 

















22 


THEORY OF RADIO RECEPTION 


106. The frequency at which the amplitude of the oscillation 
rises' and falls is equal to the difference between the frequencies 
of the two separate oscillations. For instance, if the signal os¬ 
cillation has a frequency of 10,000 cycles per second and the local 
oscillation is adjusted to a frequency of 9,000 cycles per second, 
the amplitude of the resultant oscillation rises and falls at a fre¬ 
quency of 1000 cycles per second. 

107. The desired object of producing an audible note in the 
telephones while an undamped wave signal voltage is being in¬ 
duced in the antenna can therefore be realized by the method 
above described. The manner in which this audible note is 
obtained is shown by the curves of Fig. 18 which represent, at 
A, the oscillation induced by the local high frequency oscillator. 
At the moment X, the signal oscillation shown at B is also in¬ 
duced in the antenna. 

108. The curve at C indicates the resulting oscillation. Be¬ 
fore the moment X the local oscillation induces a continuous un¬ 
damped oscillation. At the moment X the signal oscillation is 
super-imposed upon the local oscillation and the amplitude of 
the resulting oscillation alternately rises and falls at a definite 
frequency as the two oscillations come in and out of phase. 

Let us suppose, as illustrated, that the signal oscillation has 
a frequency of 10,000 cycles per second. If the frequency of the 
local oscillations is adjusted to either 9,000 cycles per second or 
11,000 cycles per second a beat frequency of 1,000 cycles per 
second is produced. 

109. The effect of rectification on the current is indicated 
at D, and the resulting telephone current at E. From these 
curves it will be plain that variations in the current flowing 
through the telephones are only produced when the signal oscil¬ 
lation is induced in the antenna and that the frequency of the 
resulting vibrations of the telephone diaphragm depends upon 
the beat frequency. The latter can be varied by varying the fre¬ 
quency of the local oscillator. As this frequency is varied, within 
certain limits, the pitch of the note in the telephone is changed. 

110. For example, to receive an undamped wave transmit¬ 
ting station which is sending on 200 meters, corresponding to a 
frequency of 1,500,000 cycles per second, the local oscillator may 
be adjusted to a frequency of 1,501,000 to obtain a beat frequency 
and audible note in the telephones of 1000 cycles per second. If 
the frequency of the local oscillator is changed to 1,500,500 the 
beat frequency and the audible note in the telephones is changed 
to 500 cycles per second. 


LESSON 3. 


INDUCTANCE—CAPACITY—RESONANCE. 

111. The simple receiving system of Fig. 14 can never be 
used in practice. The rectifier and telephones in series with 
the antenna offer such a high effective resistance that it is ex¬ 
tremely difficult for any high frequency current to flow in the 
circuit. 

112. To efficiently receive signals the circuit or circuits of 
a radio receiver must be arranged in such a way that a maximum 
current can flow for a given induced voltage. In order that the 
greatest amount of energy may be produced the effective resist¬ 
ance of the circuits must be reduced to a minimum. 

113. To undersand how this is accomplished, the effects of 
Inductance and Capacity in an electrical circuit must be fully 
appreciated. We will explain these separately. 


INDUCTANCE. 

114. Electro-magnetism: Some of the effects of electric 
currents are familiar to all. We know that they may be utilized 
for the production of heat and light. But there are other effects 
of electric currents which may be harnessed to do useful work; 
of these one of the most important is the magnetic effect. 

115. When a current of electricity passes through a con¬ 
ductor a magnetic field is built up round the conductor. The mag¬ 
netic strain is represented by lines which are called “magnetic 
lines of force.” The intensity of the 
magnetic force at different points in 
the field is measured by the proximity 
of the lines of force to each other. 

The magnetic field in and around.a 
wire carrying current is illustrated in 
the cross-sectional view of Fig. 19. 

116. The magnetic field acts in a 
definite direction and this direction de¬ 
pends upon the direction of the current 



24 


THEORY OF RADIO RECEPTION 


through the conductor. Fig. 20 shows the direction of the con¬ 
centric circles of magnetic force round a wire carrying current 
for the direction of current indicated. If the direction of the cur¬ 
rent reverses the direction of the 
magnetic field also reverses. 

117. The intensity of the field 
produced by a given current can be 
greatly increased by winding the 
wire carrying the current in the 
form of a coil. Fig. 21 shows how 
the lines of force would appear 
round a coil carrying current if the 
lines were visible. 

118. The strength of the 
magnetic field produced by 
curent flowing through a coil 
depends upon three factors: 

1. The value of the cur¬ 
rent. 

2. The number of turns in 
the coil. 

3. The “reluctance” of the 
magnetic path. 

119. “Reluctance” in a 
magnetic circuit corresponds 
to resistance in an electrical 
circuit. If the core of a coil is 
filled with iron, any given 
value of current passing through the coil sets up a very much 
stronger magnetic field than that which would be produced if 
the iron core were not present. This is due to the fact that 
iron has very much greater magnetic “Permeability” than air. 
In other words, it is much easier to set up magnetic lines of 
force in iron than in air. 

120. Law of Induced E.M.F.: If a conductor is surrounded 
by a magnetic field and the strength of the magnetic field 
changes, an e.m.f. is induced in the conductor. A change in the 
strength of a magnetic field produces, in effect, an increase or 
decrease in the number of lines of force threading the conductor 
and it is this change which induces an e.m.f. in the conductor. 
This law of induced e.m.f. underlies the operation of most elec¬ 
trical apparatus. 

121. If a number of wires are surrounded by a magnetic 
field and the strength of the field changes, an e.m.f. is induced in 
each wire. If the wires are joined together in series the total 
e.m.f. is equal to the sum of the individual e.m.f/s. It is obvious 
the same result will be obtained if a coil of wire is surrounded 
by a changing magnetic field. Each turn of the coil may be 
regarded as a separate wire in which an e.m.f. is induced. Since 
all the turns of a coil are in series with each other the e.m.f. in- 

































INDUCTANCE 


25 


duced across the entire coil is equal to the sum of the e.m.f/s 
induced in each turn. It is also evident that the value of the 
e.m.f. induced is proportional to the number of turns in the coil. 

122. The value of the e.m.f. induced in a coil is also governed 
by two other factors, viz: 

1. The extent of the increase or decrease in the number 

of lines of force threading the coil. 

2. The rapidity with which this change takes place. 

For example, if a certain e. m. f. is induced in a coil by an 

increase or decrease of 100 lines of force during one second 
an increase or decrease of 200 lines of force during one second 
will induce an e. m. f. twice as great but a change of 200 lines of 
force during two seconds will induce the same value of e. m. f. as 
a change of 100 lines during one second. 

123. In other words, the value of the e.m.f. induced in a 
coil is proportional to the rate of change in the number of lines 
of force threading the coil. 

Therefore, value of 

Induced E.M.F. = Rate of Change X Number of turns. 

124. Self-Induction: A wire or coil through which a current 
is passing is surrounded by a magnetic field produced by the 
current. If the magnetic field increases in strength new lines of 
force are formed and there is an increase in the number of lines 
of force threading the conductor. Conversely, if the magnetic 
field decreases in strength there is a decrease in the number of 
lines of force threading the conductor. 

Now, according to the rule of induced e.m.f., if there is any 
change in the number of magnetic lines of force threading a con¬ 
ductor, an e.m.f. is induced in that conductor. 

125. Therefore, if the magnetic field in and around a wire or 
coil carrying current increases or decreases in strength an e.m.f. 
is induced in the wire or coil. 

But the magnetic field in and around a conductor carrying 
current only increases or decreases in strength when the value 
of the current increases or decreases. 

126. Therefore, if the current passing through a conductor 
increases or decreases in value, an e.m.f. is induced in the con¬ 
ductor. If the current does not change there js no change in 
the magnetic field and consequently no e.m.f. is induced. 

127. The e.m.f. which is induced in a circuit by a change in 
its own current is called the e.m.f. of self-induction. 

128. Direction of self-induced e.m.f.: An e.m.f. is induced 
in a coil or wire by a relative movement between the conductor 
and a magnetic field, providing that this movement changes the 
number of lines of force threading the conductor. The effect 
can be produced by moving a conductor from a weak to a 
stronger portion of a magnetic field or by holding the conductor 
stationary and moving the magnetic field. 

129. The direction of the induced e.m.f. is governed by the 
direction of the magnetic field and the direction of motion of the 
conductor through the magnetic field. In the case of a self- 
induced e.m.f., of course, the conductor remains stationary but 


26 


THEORY OF RADIO RECEPTION 


if the current increases the lines of force cut the conductor with 
an outward direction of motion as they expand. If the current 
decreases the lines of force cut the conductor with the opposite 
direction of motion as they contract. Therefore the direction 
of a self-induced e.m.f. depends upon whether the current is in¬ 
creasing or decreasing in value. 

130. Moreover, since the direction of the lines of force them¬ 
selves depend upon the direction of current flow, the direction 
of the self-induced e.m.f. will similarly depend upon the direction 
of the current. 

131. In effect it is found that a self-induced e.m.f. is always 
in such a direction that it opposes the change of current which 
produces it. That is to say, if the current increases, the 
self-induced e.m.f. is in the opposite direction to the ap¬ 
plied e.m.f. of the circuit. This tends to decrease the effective 
e.m.f. and opposes the increase of current. On the other hand, 
if the current decreases, the self-induced e.m.f. is in the same 
direction as the applied e.m.f. and tends to increase the effective 
e.m.f. of the circuit and oppose the decrease of current. 

132. The effect of self-induction in a circuit can best be 
understood by comparison with the mechanical properties of in¬ 
ertia and momentum. 

133. It is well known that a heavy vehicle resists any effort 
to start it in motion. When it is started its speed only gradually 
increases. However, after its inertia is overcome and it has at¬ 
tained a uniform speed the only opposition to the driving power 
is the resistance of friction. So long as it continues to travel at 
a uniform speed there is no inertia. 

134. In an electrical circuit when a direct current commences 

to flow, the building up of the magnetic field in and around the 
conductor produces a self-induced e.m.f. which opposes the ap¬ 
plied e.m.f. and thereby acts to decrease the effective e.m.f. 
When the magnetic field is entirely built up the number of lines 
of force threading the conductor is constant and no further 
counter e.m.f. is induced. The current in the circuit, which is 
analogous to the speed of a vehicle, only increases gradually as 
the counter self-induced e.m.f. decreases. After a brief period it 
reaches its maximum value for a given applied e.m.f. There¬ 
after, the only opposition to the flow of the current is the resist¬ 
ance of the circuit itself. The ultimate value of the current is 
determined by Ohm’s Eaw. This ultimate value is not affected 
by the self-induction of the circuit, which only affects the initial 
value of the current when the e.m.f. is applied across the circuit. 
. 135. If a vehicle is travelling at a uniform speed there is no 

inertia. Its mass does not affect its speed. However, if the 
speed is accelerated its inertia again resists this change of speed. 

136. In an electrical circuit, if a steady direct current is 
flowing and the current is increased, either by increasing the 
applied e.m.f. or decreasing the resistance of the circuit the in¬ 
crease of current sets up new lines of force round the conductor. 
A counter e.m.f. is self-induced which resists this change of cur¬ 
rent. In a brief period of time the magnetic field and the cur- 


INDUCTANCE 


27 


rent both reach a new higher value and the effect of self-induction 
is not present, provided there is no further change in the current. 

137. Again, while a vehicle is travelling at a uniform speed 
its mass opposes any attempt to stop its motion. Its momentum 
causes it to continue in the direction in which it is travelling. 

138. In an electrical circuit, if a steady direct current is flow¬ 
ing and the source of e.m.f. is removed, the magnetic field round 
the conductor contracts and induces an e.m.f. in the circuit in 
the same direction as the applied e.m.f. As a result, the current 
does not immediately stop flowing when the source of e.m.f. is 
removed but slowly decreases to zero; provided, of course, the 
circuit is not opened by a switch. 

139. In the same way, if a steady direct current is flowing in 
a circuit and the current is reduced, either by an increase or a 
reduction of applied e.m.f., the magnetic field contracts, induces 
an e.m.f. in the conductor in the same direction as the applied 
e.m.f. and thereby acts to increase the effective voltage of the 
circuit. The self-induced e.m.f. falls to zero when the contrac¬ 
tion of the magnetic field ceases. Consequently the current in 
the circuit gradually decreases and only reaches the value de¬ 
termined by Ohm's Law when the self-induced e.m.f. falls to 
zero. 

140. The distinction between the effects of resistance and 
self-induction should be observed. Resistance obstructs the 
flow of electricity in a circuit at all times but self-induction 
merely opposes any change in the value of the current. Resist¬ 
ance absorbs energy in the production of heat—it wastes energy. 
Self-induction stores energy in the production of a magnetic field 
when the current is increasing and gives this energy back to the 
circuit when the current decreases and the magnetic field con¬ 
tracts. 

141. Value of a self-induced e.m.f.: It will be evident that 
the self-induction of a circuit is entirely due to the magnetic field 
which is produced by the current in the circuit. The self- 
induction effects are only present when there is a change in the 
current because this change of current alters the strength of the 
magnetic field. 

Therefore, the value of the e.m.f. self-induced in a coil by a 
change of current can be determined from the formula of Para¬ 
graph 123, viz.: 

Induced e.m.f. = Number of turns X Rate of change of 
magnetic lines of force. 

142. Thus the value of the self-induced e.m.f. in a circuit is 
increased by winding some of the conductor in the form of a 
coil. The greater the number of turns in this coil the greater 
is the self-induced e.m.f. The self-induced e.m.f. can also be 
greatly increased by providing the coil with an iron core because 
this increases the extent of the change in the number of lines 
of force threading the coil for any given change of current. . 

143. Now, the rate of change of magnetic lines of force which 
governs the value of a self-induced e.m.f. in any given circuit is 
manifestly dependent upon the rate of change of the current in 


28 THEORY OF RADIO RECEPTION 

the circuit. The rise or fall in strength of the magnetic field and 
the speed at which the change takes place are determined by the 
rise or fall in the strength of the current and the rapidity of the 
change of current strength. 

144. If the current is changing at the rate of one ampere 
per second a certain e.m.f. is self-induced in the coil or circuit. 
If the current changes in strength with greater speed the value 
of the self-induced e.m.f. is greater. For instance, if during 
one-hundredth of a second the current increases or decreases 
by one ampere this corresponds to a change of 100 amperes in 
one second. The self-induced e.m.f. is correspondingly higher 
in value. Again, if the current changes 2 amperes during one- 
hundredth of a second this corresponds to a change of 200 am¬ 
peres per second. The self-induced e.m.f. is still greater in value. 

145. Unit of Inductance: If any coil or circuit has induced 
in it an e.m.f. of one volt by a change of current of one ampere 
per second, it is said to have an “Inductance” of One Henry. 
The Henry is the unit of Self-Induction and from this unit we 
can measure the effects of self-induction in other coils or circuits. 

146. Thus, suppose a coil has an inductance of 5 henrys. 
This means that a change of current of one ampere per second 
induces in the coil a back e.m.f. of 5 volts. If a change of cur¬ 
rent of one ampere per second induces in a coil a back e.m.f. of 
10 volts the coil has an inductance of 10 henrys. The coil with 
an inductance of 10 henrys must have a greater number of turns 
or more iron in its magnetic path than the first coil since the 
same rate of current change induces in it a higher voltage. 

Naturally, if the current changes at a higher rate than one 
ampere per second, the back e.m.f. induced in either coil is corre¬ 
spondingly increased. 

147. Therefore, the value of the voltage self-induced in a 
coil by a change of current is equal to the product of its induc¬ 
tance and the rate of change of current, or 

Self-Induced e.m.f. (in volts) = Inductance (henrys) 
X Current change per second (in amperes) 

148. Effects of inductance in a D.C. circuit: Self-induction 
effects in a direct current circuit are only present when the cur¬ 
rent is started or stopped or when the current is increased or 
decreased. No e.m.f. is self-induced if the current does not 

change in value. Therefore, self-induc¬ 
tion does not affect the value of the cur¬ 
rent which can be determined by Ohm's 
Eaw. 

149. Current in an A. C. circuit 
Fig. 22 . . with Inductance and Resistance: A 
back e. m. f. is induced in an inductive circuit by a changing 
current. An alternating current is constantly changing in value 
and in direction. Therefore, the effects of self-induction are 
constantly present if an alternating e. m. f. is applied across 
an inductive circuit as in Fig. 22. The back e. m. f. of 

inductance in an A. C. circuit has a maximum value when the 
alternating current is changing at the greatest rate, i.e., when it 
is passing through its zero values. Conversely, the back e.m.f. 






INDUCTANCE 


29 



'S'Ox/ctAff of 
V 


of inductance is zero when the rate of current change is zero, i.e., 
when the current reaches its maximum amplitude. In other 
words, the back e.m.f. of self-induction is maximum when the 
current is zero and is zero when the current is maximum. Fig. 
23 shows the alternating current curve and the back e.m.f. curve. 
The latter is invariably 90 degrees (one-quarter cycle) behind 
the current. This will always be the relationship between the 
current in an A.C. circuit and the 
back e. m. f. of inductance. The 
phase relationship is not affected 
by the relationship between the 
applied e. m. f. and the current. 

150. Reactance: The effect¬ 

ive resistance which the back 
e. m. f. of self-induction offers to 
the flow of current in an A. C. 
circuit is called reactance or, Fig. 23 

more particularly, inductance reactance. The total resistance of 
the circuit including the reactance and ohmic resistance, is called 
the impedance of the circuit. 

151. The alternating e.m.f. applied across the circuit of Fig. 
22, which is supposed to possess inductance and resistance only, 
must be equal to the sum of the voltage drops across the induc¬ 
tance and the resistance. Therefore, the current which flows in 
an A.C. circuit possessing inductance and resistance is always 
lower than that which flows if the inductance is not present. 
The applied e.m.f. must overcome the reacting force of self- 
induction as well as the resistance of the circuit. 

152. The relation between the 
applied e. m. f., the current and 
the back e. m. f. of self-induction 
in the circuit of Fig. 22 is shown 
in Fig. 24. From this diagram 
it will be noted that the effect 
of inductance in the circuit is to 
make the current lag behind the 
voltage nearly 90 degrees. The 
Fig. 24 current curve in this diagram 

should be compared with that of Fig. 6 showing the relation be¬ 
tween current and impressed voltage in an A. C. circuit with re¬ 
sistance only, 

153. Value of Inductance Reactance: The back e.m.f. self- 
induced in a circuit depends upon the inductance of the circuit 
and the rate of current change (Par. 147). 



154. All parts of an electrical circuit possess some value of 
inductance. The inductance can be increased by including a 
coil in the circuit and the inductance of this coil depends upon 
the number of turns, its diameter, etc. If the coil has an iron 
core it has a greater inductance than if the iron is not present. 
The greater the inductance of the A.C. circuit the higher is the 
value of the inductance reactance. 






30 


THEORY OF. RADIO RECEPTION 


155. The higher the frequency of the alternations in an A.C. 
circuit the more rapid is the rate of current change and there¬ 
fore the greater is the back e.m.f. of inductance, or inductance 
reactance. 

156. We may say, then, that inductance reactance in an A.C. 
circuit is directly proportional to the inductance of the circuit 
and to the frequency of the current; the latter, of course, being 
the same as the frequency of the applied e.m.f. 

157. The value of inductance reactance in an A.C. circuit 
may be written 

6.28 X f X L 

where f = the frequency of the applied e.m.f. in cycles per 
second. 

and L = inductance of the circuit in henrys. 

The product gives, in ohms, the effective resistance of inductance 
reactance to the flow of current in an A.C. circuit. 

158. With a given value of impressed e.m.f. the effect of in¬ 
creasing either the frequency or the inductance is to increase the 
inductance reactance and decrease the value of the current. At 
zero frequency (Direct Current) the reactance is zero. At high 
frequencies the reactance is very high; the voltage is mostly 
used up in overcoming the high reactance and the current is very 
small. 

159. We have already noted that the current oscillations in 
a receiving antenna have a very high frequency. Therefore, the 
coils used.in radio receivers have a low value of inductance, 
measured in smaller units called millihenrys, microhenrys and 
centimeters. 

1 henry = 1,000 millihenrys 

1,000,000 microhenrys 
1,000,000,000 centimeters. 

160.. The coils used in radio receivers are called “induc¬ 
tance coils” or “inductances.” They are sometimes tapped and 
by means of a switch the number of active turns of the coil in the 
circuit is varied to vary the inductance of the circuit. 

. 161. It should be understood that although the coils of a 
radio receiver have a comparatively low value of inductance and 
are often composed of just a few turns of wire wound on a three 
or four-inch tube, their reactance to radio frequency currents 
may be very high. If the same coils were included in, say a 60 
cycle lighting circuit their effect would be negligible. The fre¬ 
quency of the current in the lighting circuit is so low that hardly 
any back e.m.f. would be induced in the coils. But the currents 
in radio circuits may have as high a frequency as 2,000,000 
cycles per second and consequently the reactance voltage set 
up across these small inductances may be very high. 

CAPACITY. 

162. Factors governing Capacity: In the first chapter we 
explained that the potential to which a body is raised by a given 
charge of electricity depends upon the capacity of the body; 
its ability to hold the charge. We also stated that one factor 


CAPACITY 


31 




governing the capacity of a body is its size. A given charge of 
electricity raises the potential of a small body to a higher po¬ 
tential than a large body. 

163. There are other factors which govern the capacity of 
a body. Let us suppose that an insulated plate A is charged 
to a positive potential of 20 volts and that this potential is 
measured when the plate is not in proximity to any other body. 
Now if an insulated plate B is brought near to A as in Fig. 25, 
and the potential of A is again measured it will 
be found that its potential has dropped. In 
other words its capacity has increased. More¬ 
over, it will be found that the closer the plate 
B is brought to the plate A the more the po¬ 
tential of A drops; the greater becomes its 
capacity. Then, if we insert a sheet of glass 
between the two plates we will find that the capacity 
is still greater. 

164. The increase in capacity is caused by 
an increase of the electrical straining of the space 
between the two plates. As we previously stated, 
a charged body exerts a strain on the surround-. 


n 


Fig. 25 


ing ether. When the plate B, which we will assume is at zero po¬ 
tential, is brought near the positively charged plate A the lines of 
electric strain issuing from A disturb the electrons in B. Free 
electrons in B, each representing a negative charge of electricity, 
are attracted in the direction of the positive plate A. As a re¬ 
sult of bringing the plate B within the field of A a charge of 
negative electricity is accumulated on the side of B near to A. 
This effect is described as static induction. The closer B is 
brought to A the greater is the charge induced on the former. 
Fig 25 shows the lines of electric strain which issue from A and 
terminate in the negative electrons of B. The electric field of 
A is therefore mostly concentrated in the space between the two 

P * atC 165. The strength of the charge induced on B also depends 
upon the material occupying the space between the two plates. 
This may be air, glass, mica or other non-conductor and is known 
as the dielectric. For any given spacing, if the dielectric is 
g-lass mica or other substance with high inductive capacity the 
charge induced on B is greater than if the dielectric is air 

166. As the charge on B is increased, the potential of A drops 
and consequently the capacity is increased. . , , 

167 The Condenser: The electrical condenser is based on 
the principles of static-induction. A simple form of condenser 
consists of two metal plates placed parallel to each other and 
senarated bv air, glass or other dielectric, as in Fig. 2i>. 

168. From the foregoing paragraphs it will be seen that the 

caoacitv of a condenser depends upon: , 

^ 1. The size and shape of the plates of which the con¬ 
denser is composed. . 

2. The thinness of the dielectric. 

3. The inductive capacity of the dielectric. 











32 


THEORY OF RADIO RECEPTION 


If any of these factors is increased, a larger quantity of elec¬ 
tricity must flow into the condenser to raise its potential to a 
given value or, in other words, the greater is the capacity of the 
condenser. 

169. Mechanical Analogy of Capacity : A good understand¬ 
ing of the effects of capacity in electrical circuits can be obtained 
by comparing the capacity of a condenser with the flexibility of 
a spring. 

170. If a downward 
^ j force of a definite value 

r - ' -I-J r - ‘ or*r\1iVr1 tn tVlP STVririfr 


is applied to the spring 
of Fig. 26 the spring 
will be extended a cer¬ 
tain distance until it ex¬ 
erts a force exactly 
equal and opposite to 
the applied force. The 
distance which the 
spring moves before it 
exerts a force equal and 
fyf/zec/ faze' opposite to the applied 

Fig. 26 force depends upon the 

inflexibility of the spring. 

171. Similarly if a continuous e.m.f. is applied across a con¬ 
denser a quantity of electricity will flow into the condenser until 
it exerts a pressure equal and opposite to the applied e.m.f. The 
quantity of electricity which must flow into the condenser before 
it exerts a pressure equal and opposite to the applied e.m.f. de¬ 
pends upon the capacity of the condenser. 

172. Thus, in Fig. 27, there is no difference of potential 
between the plates A and B of the condenser before the switch S 
is closed. When the switch is closed the plate A becomes 6 
volts positive with respect to B. To produce this difference of 
potential between the plates of the condenser electrons must flow 
from one plate to the other. 

The condenser then exerts an 
e. m. f. equal and opposite to 
the e. m. f. of the battery and 
the condenser is said to be 
charged. Now the quantity of 
electricity which must flow to 




Fig. 27 

charge the condenser to a difference of potential of six volts 
depends upon the capacity of the condenser. 

173. Measurement of Capacity: A condenser of unit capacity 
would be one which requires a unit charge of electricity to bring 
its plates to a potential difference of one volt. The coulomb is 
the unit of quantity (One coulomb per second equals one am¬ 
pere). Therefore a condenser which requires a charge of one 
coulomb to bring its plates to a potential difference of one volt 
is said to have a capacity of one Farad. But this unit is too 
large for ordinary purposes. The microfarad (one millionth of 












CAPACITY 


33 


a farad) is the practical unit of capacity. Very small values of 
capacity are used in radio circuits, however, and are expressed 
in milli-microfarads (one thousandth of a microfarad) and in 
micro-microfarads (one millionth of a microfarad). It is more 
common, however, to designate the capacities of condensers used 
in radio circuits in decimal fractions of a microfarad. Thus 1 mil- 
limicrofarad is commonly expressed as .001 mfd (microfarad). 

A condenser of .001 mfd. requires a charge of one billionth of 
a coulomb to charge it to a potential of one volt. In other words, 
since a current of one ampere represents one coulomb per 
second, a current of one ampere would have to flow for only one 
billionth of a second to charge a .001 mfd. condenser to a po¬ 
tential of one volt; or, similarly, if a current of one milli-micro- 
ampere flows for one second, it will charge the condenser to a 
potential of one volt. 

174. Discharge of a Condenser: Referring again to Fig. 26 , 
we have seen that if a downward force is applied to the spring 
it will extend until the opposing force of the spring equals the 
applied force. So long as the applied force remains at a steady 
value the spring will not move. 

But if the applied force is removed or decreased in value 
the tension of the spring will cause it to contract. 


C/?aryrnj Curran/ 


A/o Cumsn/ 


/fcse&nye Curran/ 


\~^1 — 

1 

°r° 1 — 


--- -- 

ff u 

V 3 

+J_ ft, 

t + 

V s 

+ ± 


f 1 

M 

\ 1 ■ 

- 

1 1 = 


Fig. 28A 28B 28C 

175. Similarly, in the electrical circuit of Fig. 28 (a), when 
the switch A is closed (B being open) a current flows which 
charges the condenser. When the condenser is charged it exerts 
a pressure equal and opposite to the applied e.m.f. and no fur¬ 
ther flow of current takes place—(see Fig. 28 [b]). 

176. But now, if the switch A is opened and simultaneously 
switch B is closed, as in Fig. 28(c) the condenser will discharge 
through the resistance. That is to say, if the e.m.f. applied 
across a condenser is removed the potential difference of the 
condenser will force a current through the circuit (providing, of 
course, a conducting path exists.) Moreover, this current will 
be in the opposite direction to the charging current. 

177. In the same way, if the e.m.f. applied across a con¬ 
denser decreases in value the higher potential difference of the 
condenser will force a current through the circuit in the oppo¬ 
site direction to the charging current until the potential differ¬ 
ence of the condenser is the same as the applied e.m.f. 

178. Current Flow in A.C. Circuit with Capacity only: If 
an alternating e.m.f. is applied across a condenser, as in Fig. 29 


















34 


THEORY OF RADIO RECEPTION 


and if the current possesses neither in¬ 
ductance nor resistance, the alternating 
current which flows in the circuit can 
be represented by the current curve of Fig. 30. 

179. The value of this current and its phase relation to the 
applied e.m.f. can best be understood by comparing the condi¬ 
tions represented by the circuit of Fig. 29 with similar condi¬ 
tions in the mechanical analogy of the movements of a spring. 

180. The e.m.f. applied across the condenser of Fig. 29 is 
an alternating e.m.f.; it is constantly changing in value and 
direction. During each alternation it rises from zero to a maxi¬ 
mum and again falls to zero. If a force of a similar changing 
nature is applied to the spring of Fig. 26, the spring will extend 

or contract as the applied force in- 
creases or decreases. The direction 
/ \ j / / 0 f movement of the spring depends 

--V-V-- j — upon whether the applied force is 

\ \ / \ / / \ increasing or decreasing. If it in- 

\ / \ creases in value, the spring moves 

N in the same direction as the applied 
force. If the applied force decreases 
Fig. 30 in value the opposing tension of 

the spring causes it to contract and the spring moves in the oppo¬ 
site direction to the applied force. 

181. As can be seen from the curves, the direction of the 
current in the circuit of Fig. 29 similarly depends upon whether 
the applied e.m.f. is increasing or decreasing. While the e.m.f. 
is increasing from zero to maximum, the current is in the same 
direction as the e.m.f. but while the e.m.f. is decreasing from 
maximum to zero, the current is in the opposite direction to the 
e.m.f. When the applied e.m.f. is decreasing the opposing e.m.f. 
of the condenser, being greater than the applied e.m.f. forces a 
current through the circuit in the opposite direction to the ap¬ 
plied e.m.f. 

182. The magnitude of the current at any moment can be 
compared with the speed of a spring when a changing force is 
applied to it. A current of electricity is measured as quantity 
per second (amperes) and speed is measured as distance of 
movement per second. 

The speed at which a spring extends or contracts depends 
upon the rate of change in the applied force and the flexibility 
of the spring. For instance, with a spring of given flexibility, 
if the applied force increases at a constant rate, the spring will 
extend at constant speed. If at any time the applied force 
ceases to increase and remains at some steady value, the spring 
will not move. The applied force and the tension of the spring, 
being equal and opposite forces, produce no movement of the 
spring unless the applied force either increases or decreases in 
value. If the applied force decreases at a constant rate, the 
spring will contract at a constant speed. But if the applied force 



Fig. 29 







CAPACITY 


35 


increases or decreases at non-uniform rate, the speed of the 
spring depends upon the rate of change in the applied force. 

183. Similarly, in an A.C. circuit with capacity only the 
magnitude of the current at any moment depends upon the rate 
of change in the applied e.m.f. and the capacity of the condenser. 
With a condenser of given capacity, the current reaches its 
maximum value when the e.m.f. is changing at its greatest rate. 
That is to say, the current is maximum when the e.m.f. is pass¬ 
ing through its zero values. The current is zero when the e.m.f. 
is at maximum because, at this point, there is no change in the 
applied e.m.f. The applied e.m.f. and the back e.m.f. of the con¬ 
denser are equal and opposite to each other. A current flows 
only when the applied e.m.f. changes in value. 

184. We can say, then, that the current which flows in an 
A.C. circuit with capacity only has a similar form to the applied 
e.m.f. but leads the e.m.f. by 90 degrees. The current is reversed 
in direction by the back e.m.f. of the condenser one-quarter 
cycle before the applied e.m.f. reverses its direction. 

185. The diagram of Fig. 30 shows this relationship between 
current and applied e.m.f. and also shows the curve of the con¬ 
denser back e.m.f. The back e.m.f. of the condenser invariably 
reaches its maximum when the current has flowed into the con¬ 
denser in one direction for the greatest length of time. That is 
to say, at the end of each alternation of current, when the cur¬ 
rent is zero, the back e.m.f. of the condenser is maximum. The 
back e.m.f. of capacity is invariably 90 degrees ahead of the cur¬ 
rent. This will be true irrespective of the phase relation be¬ 
tween the applied e.m.f. and the current. The back e.m.f. of a 
condenser in an A.C. circuit is always 90 degrees ahead of the 
current in the circuit. 

186. Capacity Reactance: The effective resistance which 
the back e.m.f. of a condenser offers to the flow of the current in 
an A.C. circuit is called the reactance of the condenser or ca¬ 
pacity reactance. 

187. The current in an A.C. circuit with capacity only is 
proportional to the capacity of the condenser and to the fre¬ 
quency of the applied e.m.f. The condenser will always take a 
sufficient charge to raise its potential to that of the applied e.m.f. 
If the capacity is large a greater quantity of electricity is re¬ 
quired to raise its potential than if the capacity is small. Since 
a current is quantity per second an increase in capacity means 
an increase in current for any given frequency of applied e.m.f. 
If the frequency of the applied e.m.f. is increased the rate of 
change of the applied e.m.f. is increased and therefore the cur¬ 
rent in the circuit is increased. 

188. Since the current is proportional to the capacity of the 
condenser and to the frequency of the applied e.m.f., it follows 
that the effective resistance or reactance of capacity in an A. C. 
circuit is inversely proportional to the capacity of the circuit and 
the frequency of the applied e.m.f. 

189. Mathematically, the capacity reactance of an A.C. cir¬ 
cuit can be written: 


36 


THEORY OF RADIO RECEPTION 


1 


6.28 X f X C 

Where f = frequency of the applied e.m.f. in cycles per 
second 

and C = Capacity of the circuit in farads. 

The quotient gives the effective resistance, in ohms, of ca¬ 
pacity to the flow of current in an A.C. circuit. 

190. In radio circuits, the currents have very high frequen¬ 
cies. Therefore, if we wish the circuits to exert any appreciable 
capacity reactance, we must make the values of the capacity very 
low. The lower the capacity, the higher is the capacity reactance 
for any given frequency. This should be remembered as we shall 
refer to it later. The condensers used for “tuning” radio cir¬ 
cuits seldom have a higher capacity than .001 mfd. Even this 
capacity may, under certain conditions, be altogether too high. 

RESONANCE. 

191. The foregoing explanations of the separate effects of 
inductance and capacity in an A.C. circuit will explain how an 
oscillating current, which is merely a high frequency alternat¬ 
ing current, can flow in the aerial of a radio receiving station. 
Direct current cannot flow in the aerial because there is no 
continuously conducting path but alternating current can flow 
because the overhead wires of the aerial and the earth below may 
be considered as the two sides of a condenser with the space 

between acting as the dielectric, 
as illustrated in Fig. 31. This 
capacity of an aerial is “dis¬ 
tributed.” It is not concen¬ 
trated in two or more plates as 
in the ordinary electrical con¬ 
denser but is distributed along 
the wires. An aerial also pos¬ 
sesses a certain value of distributed inductance. A magnetic 
field is set up round a conductor carrying current even 
although it is not wound in the form of a coil. To wind the 
conductor in the form of a coil increases its inductance but 
even when the wires are stretched out straight they possess a 
certain value of inductance. 

192. Oscillatory Circuits: An oscillatory circuit is funda¬ 
mentally the same as any other circuit with inductance and 
capacity (and all circuits possess some value of each)—but we 
have learned that the frequency of alternating current in a cir¬ 
cuit has a marked effect on the reacting voltages set up by both 
inductance and capacity. In an oscillatory circuit, the values of 
the inductance and capacity are arranged so that a high fre¬ 
quency current can flow to the best advantage. 

193. An aerial, in which the inductance and capacity are 
both distributed, is called an open oscillatory circuit, whereas a 
circuit in which the inductance is mostly concentrated in a coil 
and the capacity in a condenser is called a closed oscillatory 


D/sfr/&wfec/ 



Fig. 31 







RESONANCE 


37 


circuit. The latter is illustrated in Fig. 32. r -— 

The main difference between an open and a 8 

closed oscillatory circuit is that the open ** g 

circuit is a better radiator of waves than the |_§ 

closed circuit. When an oscillating current Fig. 32 
flows in an open circuit, it radiates waves of comparatively high 
amplitude, whereas the closed circuit radiates waves of very low 
amplitude; it is a very poor radiator. 

194. Tuning an oscillatory circuit: As we intimated at the 
beginning of this chapter, it is necessary to arrange the circuits 
of a radio receiver in such a way that a maximum current can 
flow for a given induced voltage so that the greatest amount of 
energy may be produced to operate the detecting system. We 
will now explain how this may be accomplished. 

195. We have considered separately the opposing reactions 
of inductance and capacity in an alternating current circuit. Let 


us contrast the 

CAPACITY 
The reactance of a condenser 
of given capacity in an A. C. 
circuit is decreased by an in¬ 
crease in the frequency of the 
applied e.m.f. and vice versa, 
the reactance is increased by a 
decrease of frequency. 

At zero frequency (Direct Cur¬ 
rent) the reactance of the con¬ 
denser in the circuit is infinite. 
The current is zero. As the 
frequency increases, the react¬ 
ance decreases until, at very 
high frequencies, the capacity 
reactance is very small and the 
current proportionately large. 
The reactance of a condenser 
is decreased by an increase of 
its capacity. The smaller the 
capacity, the higher is its re¬ 
actance for any given fre¬ 
quency. 

The effect of capacity is to 
make the current lead the ap¬ 
plied e.m.f. 

The reacting force of capacity 
in volts is set up 90 degrees 
ahead of the current. This 
voltage is in direct opposition 
to the voltage set up by in¬ 
ductance. See Figs. 30 and 23. 

196. How can we obtain a 
tory circuit in which both these 
which has both inductance and 


reactances: 

INDUCTANCE 
The reactance of a given in¬ 
ductance in an A. C. circuit is 
increased by an increase in the 
frequency of the applied e.m.f. 
and, vice versa, the reactance 
is decreased by a decrease of 
frequency. 

At zero frequency (Direct 
Current) the reactance of the 
inductance is zero. The cur¬ 
rent is a maximum. As the 
frequency increases, the react¬ 
ance increases until at very 
high frequencies the inductance 
reactance is very high and the 
current proportionately small. 
The reactance of a coil is in¬ 
creased by an increase of its 
inductance. The smaller the 
inductance the lower is its re¬ 
actance for any given fre¬ 
quency. 

The effect of inductance is to 
make the current lag behind 
the applied e.m.f. 

The reacting force of induc¬ 
tance in volts is set up 90 de¬ 
grees behind the current. This 
voltage is in direct opposition 
to the voltage set up by ca¬ 
pacity. See Figs. 23 and 30. 
maximum current in an oscilla- 
effects are present—in a circuit 
capacity ? 





38 


THEORY OF RADIO RECEPTION 


The answer is evident. We can obtain a maximum current 
by balancing the two reactances against each other. They act 
in direct opposition. If we can arrange the circuit so that the 
voltages set up by the inductance and capacity are exactly equal 
they completely neutralize each other. It does not matter how 
high or how low the reactance voltages may be. So long as 
they are equal they neutralize each other and the only remain¬ 
ing opposition to the current in the circuit is the resistance of 
the circuit. In other words, if we can make the two reactances 
equal, the total reactance is zero, the current is in phase with 
the applied volts and the,current in the circuit at any moment 
of time may be determined by Ohm’s Law, viz : 

Applied Volts 

Current =- 

Resistance 

197. How, then, can we arrange an oscillatory circuit so 
that these conditions are realized? If we change the inductance 
or capacity of the circuit we change the reactance but there is 
still the factor of the frequency of the applied e.m.f., which 
governs both the capacity reactance and the inductance re¬ 
actance. 

198. Let us presume, then, that the frequency of the applied 
e.m.f. is a fixed value. Applying the conditions to the antenna 
circuit of a radio receiver, we will presume that we wish to 
receive a message transmitted by a certain station on a wave¬ 
length of 400 meters, corresponding to a frequency of 750,000 
cycles per second. The e.m.f. induced in the antenna oscillates 
at this frequency. How can the circuit be arranged so that the 
capacity reactance neutralizes the inductance reactance? 

Always remembering that inductance reactance is in¬ 
creased by an increase of inductance whereas capacity reactance 
is increased by a decrease of capacity, the inductance and ca¬ 
pacity of the circuit can evidently be varied until two values 
are found at which (for the given frequency) the reactances 
are equal and neutralize each other. If the inductance is fixed 
and cannot be varied, then, of course, it will be necessary to 
adjust the capacity until the capacity reactance is equal to the 
fixed reactance of the inductance for the given frequency. 

199. When the inductance and capacity of an oscillatory 
circuit are so adjusted that the total reactance to a given fre¬ 
quency of applied e.m.f. is zero the circuit is said to be in 
resonance with this frequency. 

200. It should be understood that one is not limited to a 
single combination of inductance and capacity to bring a cir¬ 
cuit into resonance with any frequency. It is the product of 
the inductance and capacity of a circuit which determines its 
resonant frequency. For instance, a circuit with an inductance 
of 200 microhenrys and a capacity of .001 mfd will still be 
resonant at the same frequency if its inductance is increased to 
400 microhenrys and its capacity decreased to .0005 mfd. The 
product is the same in each case. 

201. It should be noted, however, that if resonance is ob- 



RESONANCE 


39 


tained with a high inductance and a low capacity the reactance 
voltages across both the inductance and the capacity, while 
neutralizing each other, both possess much higher values than 
when resonance is obtained with a low inductance and a high 
capacity. This is sometimes an important consideration in de¬ 
signing radio receiving apparatus. It is often desirable to tune 
a circuit to resonance with a certain frequency and yet choose 
such values of inductance and capacity which will make the 
neutralizing reactances at this frequency as high as possible. 
The circuit should then be tuned to resonance with as large 
a value of inductance and as small a value of capacity as pos¬ 
sible. 


202 . This is particularly important when receiving the very 
high frequencies of short waves. A very small capacity which, 
at low frequencies, has a fairly high reactance, may have a very 
low reactance at extremely high frequencies. If the capacity is 
fixed, resonance can then only be secured by reducing the re¬ 
actance of the inductance until it neutralizes the low reactance of 
the capacity. Both reactances are correspondingly low, even 
though the circuit is in resonance with the applied frequency. 

203. The Resonance Curve: Let us suppose that to receive 
a certain station the inductance and capacity of the antenna cir¬ 
cuit of a receiving system have been chosen so that the circuit 
is resonant at a frequency of 750,000 cycles. In radio parlance, 
the antenna is tuned to a wave-length of 400 meters. How 
sharply is it tuned to this wave-length? If another station is 
transmitting on 350 meters, will it be heard and cause inter¬ 
ference? 

204. If the circuit is resonant at 400 meters the reactance is 
zero at this frequency only. At the higher frequency of 350 
meters the inductance reactance is higher than the capacity re¬ 
actance and the e.m.f. induced in the aerial by the 350-meter sta¬ 
tion will encounter a positive reactance. The current will be 
smaller than at the resonant frequency. 

205 But if there is enough current to operate the detecting 
system the 350-meter station will be heard and cause inter¬ 
ference Can this be avoided? If a number of stations are trans¬ 
mitting, can we not pick out the one we wish to hear and avoid 
the interference caused by the signals of other stations. 

206 This can. to a great extent, be accomplished, as we 
shall see later. This factor is one of the most important to 
be considered in the design of a radio receiver. In New York 
Citv alone, three or four broadcasting stations are frequently 
transmitting at the same time. One is transmitting on 405 
meters; another on 455 meters; still another on 492 meters A 
receiving set must be able to select one station without inter- 
ference from the others. 

207 We can very easily determine the factors governing 
the selectivity of an oscillatory circuit by a simple experiment. 
The same method 1 can be used to determine the selectivity of any 
oscillatory circuit in a radio receiver. 


40 


THEORY OF RADIO RECEPTION 


208. The circuit of Fig. 33 
consists of a fixed inductance L, 
a variable condenser C, a vari¬ 
able resistance R, and a meter 
A, the latter being capable of 
recording the value of any os¬ 
cillating current in the circuit. 

209. An oscillator is used to induce in this circuit an oscil¬ 
lating e.m.f. of any desired frequency. We shall see later how 
an oscillator is made and how it operates. In the meantime we 
may regard it as a miniature transmitter which sends out a con¬ 
tinuous stream of undamped waves. The oscillator is provided 
with a variable condenser so that the frequency of the waves (or 
wave-length) may be constantly varied over its scale.. 

210. The variable condenser C is turned to a midway posi¬ 
tion and the variable resistance R is almost cut out of the cir¬ 
cuit. With the condenser of the oscillator at zero (which, by 
previous measurement, is known to adjust the oscillator to a 
wave-length of 200 meters) undamped waves are radiated. The 
value of the oscillating current in the circuit EC is read on the 
meter and recorded. The variable condenser of the oscillator is 
then varied from zero to maximum and readings of the current 
in EC taken from time to time to record the current in the cir¬ 
cuit. At the maximum adjustment the oscillator is radiating 
undamped waves on a wave-length of 600 meters.. By plotting 
a curve to represent the readings of the current in EC for all 
waves from 200 to 600 meters, we can very easily determine how 
selective the circuit is. 

211. A curve of this type is called a resonance curve and is 

reproduced in Fig. 34. From 
this curve it is quite evident 
that the resonant frequency of 
the circuit LC is 750,000 cycles 
per second, corresponding to a 
wave-length of 400 meters. 

212. At this frequency the 
total reactance of the circuit is 
zero, the current is in phase 
with the induced e.m.f., and is 
only limited in value by the re¬ 
sistance of the circuit. If it 
were possible to entirely elim¬ 
inate the resistance of the cir¬ 
cuit, the current at the resonant 
frequency would be infinitely 
great. 

213. From the shape ot 
this curve we can see that the 
selectivity of the circuit EC is 
fairly good. It is resonant at 
400 meters and if an e.m.f. is 
induced at a frequency corres- 





Tjt 




3 


Fig. 33 


















































































RESONANCE 


41 


ponding to 350 meters, the current is much lower. 

Effect of Resistance on Resonance Curve: Now let us 
include some of the resistance R in the circuit and repeat the 
experiment. The result is represented by the curve A of Fig. 35. 

The effects of this additional resistance in the circuit are 
evidently detrimental. Not only is the current at resonance less 
than before, but the curve is “broader.” The circuit is less se¬ 
lective. 

215. The curve B was obtained with all the resistance R in 
the circuit. This curve is even “broader” than A. The selec¬ 
tivity is very poor. If an e.m.f. is induced in the circuit by waves 
radiated on a wave-length of 350 meters, the current in the cir¬ 
cuit is almost as great as the current at the resonant frequency 
of 400 meters'. Even at the resonant frequency the current is 
quite low. 

216. It is evident that the 
curves of Figs. 34 and 35 could 
have been obtained by adjust¬ 
ing the oscillator to radiate on 
a wave-length of 400 meters 
and then varying the con¬ 
denser C of the oscillatory 
circuit from zero to maxi¬ 
mum. 

The curves, then, repre¬ 
sent the ability of this cir¬ 
cuit, with different values 
of resistance, to “select” any 
desired signal. The curves 
measure accurately its degree 
of selectivity. *■*•»*' 

217. It is quite evident from these curves that, if selectivity 
and a high value of current at resonance are desired, the re¬ 
sistance of an oscillatory circuit must be kept as low as possible. 

218. Damping: The current in an oscillatory circuit is meas¬ 
ured by its final amplitude. If an undamped oscillating e.m.f. 
is impressed upon an oscillatory circuit, tuned to resonance, the 
current builds up Until it reaches a certain maximum amplitude. 
The final amplitude is limited only by the resistance of the circuit. 
As the oscillations increase in amplitude the time is eventually 
reached when the energy dissipated by the resistance of the cir¬ 
cuit is equal to the energy supplied the circuit by the impressed 
e m f If the reaction of resistance is high, the current reaches 
its maximum amplitude in a short period of time and this final 
amplitude is comparatively low. If the resistance of the cir¬ 
cuit is small, the current takes a longer time to reach its maxi¬ 
mum amplitude and this final amplitude has a higher value. 

219. Let us suppose, however, that an undamped oscillat¬ 
ing e.m.f. is impressed on a resonant oscillatory circuit which 
has no resistance. As the circuit has zero resistance the cur- 
rent continues to increase in amplitude as long as the impressed 
e.m.f. is maintained. If the impressed e.m.f. is removed, the 
oscillations continue forever at the amplitude they had at- 



























































42 


THEORY OF RADIO RECEPTION 


tained when the impressed e.m.f. is removed; there is no re¬ 
sistance in the circuit to damp out the oscillations. 

220. But if the circuit has resistance and if, after the oscil¬ 
lations have built up to a certain amplitude, the impressed e.m.f. 
is removed, the oscillations in the circuit continue but gradually 
decrease in amplitude as energy is dissipated by the resistance 
until they are completely damped out. 

221. The rate at which oscillations in such a circuit de¬ 
crease in amplitude is called the “damping” of the circuit. 

The damping naturally depends upon the resistance of the 
circuit. If the resistance is high the damping is rapid, whereas 
if the resistance is small the damping is slower. 

222. Now, if the impressed e.m.f. is itself damped, it is 
evident that this will have the same effect on the oscillations 
as resistance in the circuit itself. If a damped oscillation is 
induced, it will have the same effect on the form of the resonance 
curve as resistance in the oscillatory circuit itself. 

It is for this reason that the reception of undamped waves 
can be effected with a very much higher degree of selectivity 
than any other method of radio communication. 

223. The extent to which damped waves affect the sharp¬ 
ness of tuning of an oscillatory circuit depends upon the damp¬ 
ing of the waves themselves. The waves radiated by some 
spark transmitters have such a high damping decrement that it 
is quite impossible to tune them sharply. Modulated continuous 
waves have the same effect on the tuning but only to a very 
limited extent. 

224. Resistance of an Oscillatory Circuit: Any factor which 
causes a loss of energy in a circuit contributes to the resistance 
of the circuit. The resistance of an oscillatory circuit mainly 
comprises the loss of energy due to 

1. The resistance of the conductor itself; 

2. Radiation of electro-magnetic waves. 

225. Conductor Resistance: The resistance of the conductor 
depends upon the composition, thickness and length of the wire. 
For all practical purposes, No. 22 copper wire, covered with one 
layer of cotton and a second layer of silk insulation, offers as 
little resistance, wound in the form of a coil or otherwise, as 
most conductors. Litzendraht wire has some advantage over solid 
wire. But, unless the radio receiver is otherwise so insensi¬ 
tive that this slight advantage is imperative, the use of Eitzen- 
draht wire is not necessary. Too much distributed capacity in 
an inductance coil will increase the resistance of a circuit, es¬ 
pecially if the coil is not shunted by a variable condenser to 
tune the circuit to resonance. Every coil has some value of 
distributed capacity but it should be kept as low as possible. It 
is usually unnecessary to use shellac on a single layer solenoid. 
This increases the distributed capacity and the resistance. 

Scraping contacts, instead of pig-tail connections, are a more 
frequent cause of conductor resistance than any other part of 
the oscillatory circuit of some radio receivers; and an unsoldered 
joint in the wiring of a receiver may have greater resistance than 
a coil smothered in shellac. 


RESONANCE 


43 


226. Resistance of an Iron-core Coil: At high frequencies 
an inductance coil with an iron core has a greater effective re¬ 
sistance than an air-core coil. This resistance is due to eddy- 
current losses in the iron core. These eddy-currents also have 
the effect of reducing the permeability of the iron at high fre¬ 
quencies so that the inductance of the coil is less than it should 
be. Eddy-currents can be partially eliminated by making the 
laminations of the iron core very thin and insulating them well 
from each other. 

227. Radiation Resistance: The radiation of electro-mag¬ 
netic waves is just as much a loss of energy and therefore a 
factor of resistance as conductor losses in an oscillatory cir¬ 
cuit. As we have already noted, an aerial is a very much better 
radiator of waves than a closed oscillatory circuit. The aerial, 
therefore, has a greater resistance than the closed circuit. The 
resistance of an antenna is also high because of its distributed 
capacity. Its resistance is increased by a poor ground con¬ 
nection. 

228. A Simple Receiving Circuit: Fig. 36 shows a simple 
receiving circuit designed according to the principles of resonance 
we have learned in this chapter. The aerial of the receiving sta¬ 



tion is tuned to resonance with any desired frequency by in¬ 
serting a variable inductance, in the form of a tapped coil, and a 
variable condenser between the aerial and the ground. A suit¬ 
able maximum value for the variable condenser is .001 mfd. 

229. The value of the inductance coil depends upon the 
wave-length range it is desired to cover. To tune the aerial to 
wave-lengths from 200 meters to 600 meters the coil may be 
wound with 100 turns of wire on a 3 or 4-inch tube and tapped 
every 20th turn. The inductance taps are used for coarse tun¬ 
ing and the variable condenser for fine tuning. 

230. For reasons already given, the rectifier and telephones 
cannot be included directly in the antenna circuit. The detect¬ 
ing circuit can, however, be connected across the inductance 
coil as shown in Fig. 36. The reacting voltage which is set up 
across the terminals of this coil by the oscillations in the aerial 
act upon the detecting circuit and signals are detected. . # 

This arrangement constitutes one of the simplest receiving 
systems with any degree of efficiency. 








LESSON 4. 


CURRENTS IN COUPLED CIRCUITS. 


231. The simple receiving circuit of Fig. 36 has several dis¬ 
advantages. In the first place, the detecting system is only con¬ 
nected across a portion of the inductance of the antenna circuit. 
Full advantage is not being taken of all the energy in the antenna 
circuit. The most undesirable feature of this arrangement, how¬ 
ever, is its lack of selectivity. The resistance of an open oscilla¬ 
tory circuit is quite high and consequently sharp tuning cannot 
be obtained. The resonance curve of such a circuit is quite broad. 

232. These disadvantages may be partially overcome by 
connecting the detecting circuit across a closed oscillatory cir¬ 
cuit and “coupling” the closed circuit to the open antenna cir¬ 



cuit. One such arrangement is shown in Fig. 37. If the energy 
in the antenna circuit can be transferred to the closed circuit 
and the latter tuned to resonance, greater audibility and selec¬ 
tivity are made possible. The detecting system is connected 
across all the inductance and capacity of the closed circuit and 
therefore takes advantage of all the energy in the circuit. More¬ 
over, the resistance of the closed circuit is considerably less than 
that of the antenna circuit. 

233. In this chapter we will discuss the methods of trans¬ 
ferring energy from one circuit to a second circuit and the effect 
of this transfer upon the currents in the circuits. 

234. Different Kinds of Coupling: When energy is trans¬ 
ferred from one circuit to another the circuits are said to be 
coupled. There are different types of coupling, the most im- 








COUPLED CIRCUITS 


45 


portant being inductance and condenser coupling. In the former, 
part of the magnetic field set up by currents in the system is 
common to both circuits. In the latter, part of the electro-static 
field is common to both circuits. The first is called magnetic 
or inductive coupling and the second is called static or capacitive 
coupling. 

235. If, as in Fig. 38, energy in the tuned circuit LI, Cl, 
M is transferred through the common inductance M, to the tuned 
circuit L2, C2, M, the common inductance M is used to transfer 
the energy and the magnetic coupling is said to be direct. 



c. 


Fig. 38 


HI- 


C *2 



Fig. 39 



Cs 

- II - 

HI- 


c, c* 


Fig. 40 


c, 


i > 

Hh 

- r mm^ 

< i 

-HI— 

Cg. 

Hh 

—II— 


Fig. 41 


236. If, as in Fig. 39, energy in the tuned circuit LI, L2, Cl 
is transferred to the tuned circuit L3, L4, C2 through that part 
of the magnetic field, set up around L2 and L4 which is com¬ 
mon to both circuits, the coupling is inductive, but is not direct. 

237. In Fig. 40 energy in the tuned circuit L2, Cl, C3 may 
be transferred to the tuned circuit L2, C2, C3, through the com¬ 
mon capacity C3. The two circuits are coupled capacitively. . 

238 Fig. 41 shows another example of capacitive coupling 
in which the condensers Cl and C2 are the coupling capacities. 

239 Inductive coupling is most commonly used to couple 
the circuits of a radio receiver. We will explain this type of 


coupling in some detail. . , 

240. Mutual Induction: According to the law of induced 
e m f (Par 120) an e.m.f. is induced in a conductor if it is sur¬ 
rounded by a magnetic field and the strength of the magnetic 
field changes We have already noted that this has the effect 
of self-inducing an opposing e.m.f. in a coil carrying a changing 


241 But an e m.f. is induced in a conductor which is sur- 
rounded by a changing magnetic field, even if the field is not be¬ 
ing produced by current passing through the conductor itself 
but through another conductor in proximity to the first. 





























46 


THEORY OF RADIO RECEPTION 


242. For instance, if current is flowing through the coil 
LI of Fig. 42, some of the magnetic field produced by this cur¬ 
rent interlinks with the coil L2 in proximity to LL If the cur¬ 
rent in LI increases, the magnetic field expands as the new lines 
of force developed by the increase of current intensify the mag¬ 
netic field. The coil L2, 
without any movement on 
its own part, enters a 
stronger portion of the 
magnetic field and is 
threaded by an increased 
number of lines of force. 
Consequently an e.m.f. is 
induced in L2. If its 
terminals are connected 
across a circuit, a current 
will flow in the circuit. 

243. When a change in 
the strength of the mag¬ 
netic field set up by current in one coil induces an e.m.f. in a sec¬ 
ond coil in proximity to the first, the induced e.m.f. is described 
as the e.m.f. of mutual induction. 

244. Value of e.m.f. Induced by Mutual Induction: This 
value depends upon factors similar to tne e.m.t. of self-induction. 
The voltage self-induced in a coil by a change of current is equal 
to the product of its inductance and the rate of current change 
(see Par. 147). 

In the same w;.y, the voltage induced in a coil by a change 
of current in another coil is proportional to the rate of current 
change in the latter and to the mutual induction of the two coils. 

245. Mutual induction is measured in the same units as self- 
induction, viz: henrys, millihenrys, etc. If the mutual induction 
of two coils is 2 henries, this means that a current change of 
one ampere per second in the first coil induces an e.m.f. of two 
volts in the second coil. 

246. Mutual induction evidently depends upon the number 
of turns in each of the two coils and upon their position with 
respect to each other. If the two coils are widely separated, 
the mutual induction is low. Similarly, if one coil is turned 
at right angles to the other, the mutual induction is very low. 

247. By rotating one coil inside or alongside another it is 
possible to vary the mutual induction of two coils from a low 
minimum to a certain maximum, the latter depending upon the 
number of turns on each of the coils and their distance apart. 

248. Coefficient of Coupling: The Coefficient of coupling 
defines the relationship between the mutual induction of two 
circuits and the total self-induction of the circuits themselves; 
in other words, it defines the extent to which the circuits are 
coupled. Suppose two circuits are coupled by mutual induction 
and the self-induction of each of the two circuits, separately, is 
ten millihenrys. Then if the mutual induction is also ten milli- 





































COUPLED CIRCUITS 


47 


henrys the coupling between the circuits is one hundred percent. 
One hundred percent coupling can only be obtained if the total 
magnetic field of the two circuits is common to both circuits. 
If, in the instance given above, the mutual induction is only five 
millihenrys, the coupling is 50 percent; or if the mutual induc¬ 
tion is two millihenrys, the coupling is 20 percent, etc. 

249. The relationship between the mutual induction and the 
total self-induction of two circuits which determines the co-ef¬ 
ficient of coupling between the circuits can be written 

M 

k = - 

VLlxL2 


where k = coefficient of coupling 

M = mutual induction between two circuits 
LI = Total self-induction of first circuit 
L2 = Total self-induction of second circuit. 

Thus, in the case given above of 100 percent coupling, if M, LI 
and L2 are each 10 millihenrys, then 
10 

k =-= unity (100 percent) 

V 10 x 10 


but if M = 5 millihenrys then k = .5 or 50 percent coupling. 

250. Direction of Induced e.m.f.: The direction of an e.m.f. 
induced in a coil depends upon the direction of motion of the 
magnetic field across the coil and the direction of the magnetic 
field itself (Par. 129). The direction of the field, of course, de¬ 
pends upon the direction of the current through the coil which 
is setting up the magnetic field. 



Cu/tRCHT IN L / / HCRCRHH* < ^ W " * 


Fig . 43 


Fig. 44 


251. For instance, if the current in LI, Fig. 43, increases, 
the magnetic field expands and cuts the coil L2 with an out¬ 
ward motion. An e.m.f. is induced in L2 and if L2 is connected 
across a circuit, a current flows through the circuit in the op¬ 
posite direction to that of the current through LI. 

252. If the current in LI decreases, the field contracts and 
cuts the coil L2 with an inward motion. But the direction of 
the current in LI has not changed. Therefore the current through 
L2 is in the same direction as the current through LI (See 

Fig. 44). , . . J J 

If the current in LI does not change, no e.m.f. is induced 

in L2. 






















THEORY OF RADIO RECEPTION 


48 


253. All other factors being equal, the value of the e.m.f. 
induced in L2 depends upon the rate of change of the current 

^2*54. It is evident, then, that if the current through LI al¬ 
ternately increases and decreases, an alternating e.m.f. will be 

induced in L2. . 

For instance, if, by a variation of the resistance of. the pri¬ 
mary” circuit in which LI is connected, the current in LI in¬ 
creases and decreases in the manner indicated in Fig. 45A, the 
primary current never changes its direction but the variations have a 

form similar to that of an 
alternating current. Fig. 45 B 
shows the form and direction 
of the e.m.f. which is induced 
in L2 by these variations of 
the primary current. 

255. It will be noticed 
that the e.m.f. induced in L2 
is an alternating e.m.f. and 
its form is exactly similar to 
the form of the current vari¬ 
ations in the primary; the 
e.m.f. alternations' in L2, 
however, lag 90 degrees be 
hind the primary current 
variations. This can be ex¬ 
plained as follows: 

Fig. 45, the current in LI is 
decreasing. Therefore the e.m.f. in L2 is in the same direction 
as the primary current. The value of this e.m.f. depends upon 
the rate of current change in the primary. The primary cur¬ 
rent changes at the greatest rate at the moment Y and the e.m.f. 
in L2 is therefore at its highest value at that moment. At the 
moments X and Z the current in the primary is neither increas¬ 
ing nor decreasing; at these moments the e.m.f. in L2 is zero. 
Up to the moment X the current in the primary is increasing. 
Therefore during this time the e.m.f. in the secondary is in the 
opposite direction to the primary current. 

257. We can say, then, that if 
two circuits are coupled induct- n 
ively and a varying direct current //7 
flows in the primary circuit, an 
alternating e.m.f. is induced in the 
secondary; the e.m.f. alternations 
have the same form as the current 
variations in the primary but lag £ 

90 degrees behind these variations. //7 

258. It can also be shown that 
if two circuits are coupled induct¬ 
ively, an alternating e.m.f. is in¬ 
duced in the secondary circuit if an 
alternating current flows in the 




256. During the time X to Z, 


r/M£ 

























COUPLED CIRCUITS 


49 


primary. 

259. Fig. 46, A shows the curve of the alternating current 
in the primary and the resulting e.m.f. induced in the secondary 
is shown at B. In explanation of this diagram, consider the 
e.m.f. alternation in the secondary circuit from the moment X 
to the moment Z. From X to Y the current in the primary is 
decreasing and is in a positive direction. The e.m.f. in the sec¬ 
ondary is therefore also in a positive direction during this period. 
From the moment Y to the moment Z the current in the primary 
is increasing but it is now in a negative direction. Therefore 
the e.m.f. in the secondary is still in a positive direction. The 
rate of current change in the primary is greatest when it is 
passing through its zero values; therefore the e.m.f. induced in 
the secondary is maximum at these moments. On the other 
hand, the rate of current change in the primary is zero when 
it is maximum; therefore the e.m.f. induced in the secondary is 
k zero at these moments. 

260. We can say, then, that if two circuits are coupled induc¬ 
tively and an alternating current flows in the primary circuit, an 
alternating e.m.f. is induced in the secondary; the e.m.f. alterna¬ 
tions have the same form as the current in the primary but lag 
90 degrees behind the primary current alternations. 

261. In either of the two instances cited above in Para¬ 
graphs 257 and 260, an alternating current will flow in the sec¬ 
ondary circuit, provided that the impedance of the secondary 
circuit is low enough to allow a current to flow. The impedance 
offered by the secondary circuit, of course, depends upon its 
inductance, capacity and resistance as well as the frequency 
of the induced e.m.f. If, for a given frequency of induced e.m.f. 
the impedance is high enough, no current will flow in the sec¬ 
ondary circuit; only e.m.f. alternations will be set up across it. 

262. Effect of Coupling Upon Resonance Curve: When con¬ 
sidering the effect of coupling upon the resonance curve of 
coupled circuits, a great many factors have to be taken into con¬ 
sideration. For instance, if two circuits are inductively coupled 
and an alternating current flows in the secondary circuit as a 
result of the e.m.f. induced by a changing current in the primary, 
the current in the secondary circuit induces an e.m.f. back in 
the primary circuit. The e.m.f. induced in the secondary by 
the primary current lags 90 degrees behind the primary current. 
Similarly the e.m.f. induced in the primary by current in the 
secondary lags 90 degrees behind the secondary current. If the 
secondary is a tuned circuit the secondary current may lead the 
voltage induced in it; it may be in phase with the voltage or 
it may lag behind the voltage, according to the frequency of 
the induced voltage. 

263. There are other effects which must be considered but 
it is evident that the interactions between two coupled circuits 
are very complicated and require a very lengthy explanation 
which is beyond the scope of this work. In any case, it may be 
better understood if we merely explain the actual effects in 
practice. 


50 


THEORY OF RADIO RECEPTION 


264. We will demonstrate the effect of coupling upon the 
resonance curve of two inductively coupled circuits when each of 
the circuits is tuned to the same resonant frequency. This is 
the most common type of coupling in the high frequency cir¬ 
cuits of a radio receiver. For example, in Fig. 37, the antenna 
circuit is inductively coupled to the closed circuit E2 C2, and 
to receive signals each circuit is tuned to the frequency of the 
signal oscillation. 

f 265. Fig. 47 represents the same arrangement of circuits. 
The primary tuned circuit El, L2 Cl is coupled to the second¬ 
ary tuned circuit E3 C2 by the mutual inductance between the 

coils E2 and E3. By means of 
the condensers Cl and C2 the 
circuits are separately tuned 
to the same resonant frequency 
and the resonance curve of either 
circuit is represented by the dia¬ 
gram of Fig. 48. 

The mutual inductance be¬ 



Fig. 47 


tween L2 and L3 is such that the coeficient of coupling of the 
circuits is, say .35, or a coupling of 35 percent. In the secondary 
circuit is connected a high frequency current measuring meter A. 

266. Now if, by means of an oscillator, we induce in the 
primary circuit an undamped oscillating e.m.f. and vary the 
frequency of this e.m.f. from below the resonant frequency 
of either circuit to above the resonant frequency and note the 
readings of the meter A from time to time we can plot a curve 
which will show the effect the coupling of the circuits has upon 
the resonance curve of the secondary circuit. 

267. Fig. 49 shows the resulting curve together with the 
resonance curve of Fig. 48 as a dotted line for comparison. It 
can be seen from the curve that there are two frequencies of 
applied e.m.f. at which the total reactance is zero; in other 
words, there are two resonant frequencies, one lower and the 
other higher than the fundamental frequency of the circuit by 
itself. These two frequencies are shown by the peaks in the 
resonance curve. A similar curve can be obtained if readings 
are taken of the primary current against frequency. 

268. It should be clearly understood that this curve does not 
show that the current in 
either primary or secondary 
circuit oscillates at two dif¬ 
ferent frequencies when an 
oscillating e.m.f. is induced 
in the primary circuit. It 
shows that a forced oscilla¬ 
tion at the frequency of the 
applied e.m.f. flows in the 
circuits and that there are 
two frequencies of applied 
e.m.f. at which the current 
in either circuit is a maxi¬ 
mum. 














COUPLED CIRCUITS 


51 


269. It is evident, then, that if two tuned circuits ate 
coupled closely, the system has a broad resonance curve with 
two peaks. If the re¬ 
sistance of either circuit 
is fairly high the peaks 
will be flattened out con¬ 
siderably, resulting in a 
very broad, flat resonance 
curve. 

270. Obviously these 

conditions are unsuitable 
for the circuits of a radio 
receptor if selectivity is 
desired. Sharp tuning 
cannot possibly be ob¬ 
tained with such a broad Fig 49 

resonance curve. Moreover, the amplitude of the current is 
quite low. 

271. Now if the coupling between the two tuned circuits of 
Fig. 47 is reduced, by decreasing the mutual inductance, and a 
new resonance curve is then plotted, its form will be similar to 
the curve A of Fig. 50. It will be seen from this curve that 
as the coupling is decreased the two resonant frequencies are? 
less widely separated. 

If the coupling is again reduced the two frequencies ap¬ 
proach the natural frequency of each circuit, until, at some low 
value of coupling, the two frequencies merge into one—the nat¬ 
ural frequency of either circuit (See Curve B). 

272. The curves of Figs. 49 and 50 plainly demonstrate the 
effect which coupling has upon the selectivity of a radio r t 2 

ceiver with inductively 
coupled circuits. For iff J 
stance, if the secondary 
closed circuit of the re¬ 
ceiving system of Fig. 37 
is closely coupled to the 
open antenna circuit, the 
tuning will be very broad 
and the greater the resist¬ 
ance of either circuit, the 
Fluency 50 broader will the tuning be¬ 

come. With tight coupling selective reception is impossible. 

273. If the mutual induction between the two circuits of 
Fig. 37 is reduced, the coupling is reduced and the more the 
coupling is reduced the more selective will the receiving system 
become. 

274. But what effect will this reduction of coupling have 
upon the strength of signals in a receiver with inductively 
coupled circuits? The signal strength—or audibility—is pro¬ 
portional to the amplitude of the currents in the circuits. 

Let us see, then, how the amplitude of the current in the 
secondary of two coupled oscillatory circuits is affected by a 









52 


THEORY ©F RADI© RECEPTION 


variation of coupling. 

275. If the coupling of the two circuits of Fig. 47, each tuned 
to the same resonant frequency, is varied from zero to 40 percent, 
while an undamped e.m.f. of constant frequency is induced in 
the first circuit by the oscillator, we can plot a curve to show the 
relative strength of the current in the secondary circuit for dif¬ 
ferent values of coupling. The result is given in Fig. 51. It 



will be seen from this curve that in this case the secondary 
current is a maximum with a coupling coefficient of .09. As the 
coupling is increased beyond or decreased below this value the 
current in the secondary decreases. 

276. When two circuits (each tuned to the same resonant 
frequency) are inductively coupled, it will invariably be found 
that, for a given resistance of circuits, there is one low value 
of coupling at which the current in the secondary is a maximum. 
This low value of coupling depends upon the resistance of the 
circuits. For instance, if the resistance of either or both the 
circuits of Fig. 47 is increased, a closer coupling than .09 is neces¬ 
sary to obtain a maximum current in the secondary. The 
resonance curve is correspondingly broader. On the other hand, 
if the resistance of the circuits is decreased, a looser coupling 
than .06 is necessary to obtain a maximum current in the sec¬ 
ondary; the resonance curve is correspondingly sharper. 

277. Transformer: A transformer consists of two coils placed 
in such a relation to each other that a change of current in one 
coil induces an e.m.f. in the other. The coil in which the orig¬ 
inal change of current takes place is called the primary and 
the second coil is called the secondary. A transformer is used, 
of course, to couple two circuits inductively. The design of a 
transformer depends upon the purpose for which it is intended 
and the frequency of the currents to be carried by the trans- 




























































































COUPLED CIRCUITS 53 

former. In Lesson 7 more details are given of the trans¬ 
formers used to couple the circuits of a radio receiver. 

278. The coils LI and L2 of Fig. 37 constitute a transformer 
to inductively couple the antenna and secondary circuits of this 
receiving system. From the foregoing paragraphs of this chap¬ 



ter it is evident that, to adjust this circuit for maximum sensi¬ 
tiveness, it should be possible to vary the coupling between the 
antenna and secondary circuits. The exact low degree of coup¬ 
ling which gives maximum audibility depends upon the resist¬ 
ance of the two circuits and some variation of coupling is neces¬ 
sary to control the selectivity of the system. 

279. Fig. 52 shows a transformer with variable coupling, 
known as a vario-coupler, which is used in the inductively 



Fig. 53 


54 


THEORY OF RADIO RECEPTION 


coupled receiving system of Fig. 37. The outside coil of this 
coupler is connected in series with the antenna to form the coil 
El of Fig, 37. Taps are made so that the antenna circuit may 
be tuned to resonance by varying the inductance. The inner coil 
of the coupler forms the inductamce of the secondary closed cir¬ 
cuit. A variable condenser is connected across its terminals. 
This inner coil is called the rotor as it can be revolved by means 
of the shaft to which it is attached. The turning of this rotor 
varies the mutual induction between the coils and thereby varies 
the coupling between the two circuits. 



Fig. 54 

-rgne>? 

Fig. 53 shows another type of coupler which is called 
a Multi-Range” coupler as the primary is wound with sufficient 
turns of wire to tune the antenna to receive signals from 200 to 
3,000 meters. If this coupler is used with a crystal detector an 
additional loading coil is required in the secondary circuit to 
tune the secondary to the long waves. The complete circuit 
is given in Fig. 54. 












LESSON 5. 


5IOHHT 


DS 


ndiff orit dl .giT nl 
[J ;cori rioirfw tmn 



281. Modern methods of radio transmission and reception 
are all based on the functioning of the three electrode vacuum 
tube. Improvements are constantly made. New circuits are 
developed. There are innumerable ways in which one or more 
vacuum tubes can be used in both transmitting and receiving cir¬ 
cuits. But in every case the operation of the circuit is based on 
the functioning of the vacuum tube itself. The tube has well 
been called the “heart” of modern radio. 

tiilq sril tadnnoD 9W 
- n/amenf ? ,-giT ni as 
f r q .Jrtomsift 9fit 

// riT .dnarmslft sdJ 
>lq ot bsioEiJ 
U hr jkmb Dlciq 

o snlsV .c8£ 

[ orl} noqn abnaq 
'i lo ainlBiaqmal 
} lo 9nii>v srit oJ 
o ogBtlov orii 11 
irfw tB klfii orfi 
rjjoq odt lilnu 
*T9D x> aodoB3i 

282. Fig. 55 illustrates a typical vacuum tube together with 
the signs which are used to represent the three electrode tube, 
or triode, in wiring diagrams. The filament of the tube is 
similar to the filament of an electric light bulb. Two other ele¬ 
ments, known as the plate and the grid, are included in the tube. 
The size and shape of these three elements vary in different 
types of tubes. In one type, as represented, the plate is a cylin¬ 
drical piece of metal which encloses both the grid and filament. 
The grid resembles a small spring in the center of which is 
suspended the filament. The three electrodes are enclosed in a 
glass envelope. Air is excluded from the space inside producing 
a very high vacuum. 

oJelq 
> srfct 
1 



Fig. 55 


THEORY OF OPERATION. 


283. Filament Circuit: When a current passes through the 
filament of a tube it is heated and incidentally produces light. 




























56 


THEORY OF RADIO RECEPTION 



In Fig. 56 the Filament or “A” battery is the source of the cur¬ 
rent which heats the filament. The variable resistance, called 
the filament rheostat, controls the value 
°f the current passing through the fila- 
l ) ment and thus regulates its tem- 

y perature. The object of passing cur¬ 
rent through the filament is not 

to produce light but it has been 
Rheostat found that electrons are emitted 
from some metals in vacua when 
their temperature is raised above 

a certain degree. Increase of tem¬ 
perature usually promotes this emission 
Fig. 56 of electrons. 

284. Current in Plate Circuit: It is possible to attract the 
negative electrons emitted by the filament to the plate of the 
tube if the plate is raised to a positive potential with respect to 
the filament. As we have learned, one of the elementary laws 
of electricity is that unlike charges attract each other. Thus, if 
we connect the plate of a tube to the positive terminal of a bat¬ 
tery, as in Fig. 57, and connect the negative end of the battery to 
the filament, the plate is at a positive potential with respect to 
the filament. The electrons emitted by the filament are at¬ 
tracted to the plate and a current flows in the circuit of Bl, or 
plate circuit as it is called. 

285. Value of Plate Current: The value of this current de¬ 
pends upon the potential to which the plate is charged and the 
temperature of the filament. There is, however, an upper limit 
to the value of the current for a given temperature of filament. 
If the voltage of the battery Bl, Fig. 57, is steadily increased, 
the rate at which electrons are attracted to the plate increases 
until the potential difference between the plate and filament 
reaches a certain value. Beyond 
this value of Bl the plate current 
does not increase. To increase 
the plate current the temperature 
of the filament must be raised. 

Again, however, a limit is reached. 

286. The reason for this sat¬ 
uration value of the plate cur¬ 
rent is explained by the fact that 
the electrons occupying the space 
between the filament and the plate 
themselves constitute a negative 
charge is called the “space 



Fig. 57 

charge of electricity. This 
, - , . * . charge” of the tube. The space 

charge, being negative, assists the negative electrons near the 
plate in their movement towards the plate but tends to repel 

LSrp!r ng ^ fiIament Md retards their “ovemLt 

, 287 v Unilatera 1 conductivity of tube: If the filament circuit 
is opened the filament is cold and no electrons are emitted. The 












VACUUM TUBE DETECTOR 


57 


space between the filament and the plate does not ©onduct cur¬ 
rent. Therefore, no current can flow in the plate circuit. It is 
important to note, however, that even when the filament is 
heated and electrons are emitted, the space between the filament 
and the plate will only conduct current in one direction. That 
is to say, the electrons emitted by the filament will only be at¬ 
tracted to the plate if it is positively charged with respect to the 
filament. If it is negatively charged the negative electrons 
emitted by the filament will be repelled by the like charge on the 
plate. 

288. Therefore, if a source of alternating e.m.f. is connected 
in place of the battery B1 of Fig. 57 current only flows in the 
plate circuit when the direction of the alternating e.m.f. raises 
the plate to a positive potential with respect to the filament. 
During the negative alternations no current flows. In other 
words, the A.C. current is rectified. 

289. A two electrode tube (with filament and plate only) 
can be used to rectify alternating currents and was formerly 
used as the rectifier of a radio detecting system. For the latter 
purpose, however, it has been superceded by the three electrode 
tube. 



HUi 


+ — 


290. The grid: The principles underlying the use of the 
third electrode, or grid, can best be understood by investigating 
the effect on the current through a tube when the potential of 
the grid with respect to the 
filament is varied above and 
below zero. This test can 
be made with the arrange¬ 
ment of apparatus shown in 
Fig. 58. 

291. In this circuit, the 3s 

plate is held at a fixed po- 
tential with respect to the fil- 
ament by the battery VWWW'MAAA 
The filament temperature is 

also fixed by the battery B3.I_ |||h __ lilil (v) 

However, the potential of +1 1 - +1 1 T 

the grid with respect to the 
filament may be varied from 
a certain negative potential 
to a certain positive poten- g ' 

tial by moving the sliding contact along the potentiometer resist¬ 
ance which is connected with the grid battery B2. When the 
contact is at the exact center of the potentiometer the grid is 
at zero potential in relation to the negative end of the filament. 
If the contact moves towards the positive end of B2 the grid is 
at a positive potential to the filament and if the contact moves 
from center towards the negative’ end of B3 the grid is at a 
negative potential. 


“®— 


















58 


THEORY OF RADIO RECEPTION 


292. If this diagram is carefully considered, it will be seen 
that there are now two circuits through the tube—the plate cir¬ 
cuit and the grid circuit. The former consists of the plate, the 
battery B1 and the filament. So long as electrons pass from 
the filament to the plate a current flows in this circuit, the bat¬ 
tery B1 being the source of e.m.f. The value of the plate cur¬ 
rent at any time can be read on the current measuring meter Ap. 
The grid circuit consists of the grid, the potentiometer and the 
filament. If the grid is at a negative potential in relation to the 
filament little or no current can flow in this circuit but if the grid 
is positive electrons are attracted to the grid and a current flows, 
the grid battery B3 being the source of e.m.f. The value of the 
grid current at any time may be ascertained from the meter Ag. 
The voltage of the grid with respect to the filament, for any 
position of the potentiometer contact, can be measured by the 
voltmeter V. 

293. Effect of grid potential on plate current: It should be 
fairly evident that the voltage of the grid has a considerable 
effect upon the value of the plate current. We have already 
seen that an increase of plate potential increases the plate cur¬ 
rent up to a certain limit. When the plate potential is raised 
it tends to neutralize the space charge and thereby increases the 
current. The raising of the potential of the grid has the same 
effect. However, since the grid is between the filament and 
plate a given change of grid potential has a much greater effect 
upon the space charge than the same change of plate potential. 
It follows that a given change of grid potential has a much 
greater effect upon the plate current than the same change of 
plate potential. For example, an increase of 2 volts grid poten¬ 
tial may increase the plate current to the same extent as an in¬ 
crease of 10 volts plate potential. 

294. To increase the plate current the grid need not actually 
be raised to a positive potential with respect to the filament. 
For instance, if the grid is at a negative potential of 2 volts and 
its potential is raised to 1 volt, the plate current is increased 
although the grid is still negative to the filament. For a given 
temperature of filament and plate potential the raising of the 
grid potential, however, does not increase the plate current be¬ 
yond a definite saturation point. 

295. The lowering of the grid potential has the same effect 
as lowering the plate potential but, for the reasons above, a very 
small decrease in the grid potential may greatly decrease the 
plate current. If the grid is lowered to a negative potential with 
respect to the filament some value of grid potential will be 
reached which completely repels the electrons emitted by the 
filament so that none can reach the plate. The plate current 
will then be reduced to zero. 

296. The actual effect upon the plate current of a tube by 
any given variation of grid potential depends upon the temper¬ 
ature of the filament, the plate potential and the construction of 
the tube. 

297. Characteristic Curve: If we move the sliding contact 


VACUUM TUBE DETECTOR 


59 


of the potentiometer of Fig. 58 so that the grid voltage varies 
from some negative value at which the plate current is zero to 
some higher value at which the plate current is maximum and 
plot a curve showing the values of the plate current as the grid 
potential is varied we obtain what is known as the “character¬ 
istic” curve of a tube. 

. 298. One curve of this type is shown in Fig. 59. It will be 
noticed in this case that when the grid potential is 2 volts nega¬ 
tive the plate current is zero. As the grid voltage is increased 
the plate current increased 
slowly at first, then more rap¬ 
idly and at a constant rate of 
increase. At a grid potential 
of from 2 to 3 volts positive the > 
rate of increase is non-uniform c 
and the plate current reaches a s 
maximum value when the grid 
potential is 3 volts positive. 

299. The grid current, as 

shown, is negligible while the 
grid potential is negative. An 
appreciable current only flows 
in the grid circuit when the Fig. 59 

grid becomes positive to the filament. 

300. It will be noted that the plate current curve, chiefly due 
to the effect of the space charge, is not a straight line. There is 
a lower and upper bend joined together by a fairly symmetrical 
linear portion. 

301. The characteristic curve of a triode invariably takes a 
form similar to this. The actual values of grid potential and 
plate current, as measured on the horizontal and vertical axes 
of the diagram, depend upon the three factors we have men- 
tioned—the filament temperature, the plate potential and the 
construction of the tube itself—but in every case the general 
shape of the plate current curve is similar to the curve of Fig. 59. 

THE VACUUM TUBE AS A DETECTOR. 

302. One method of using the triode as the rectifier of a 

radio receiver can be explained by means of the circuit of Fig- 
60 The plate of the tube in this circuit is held at a positive 
potential by the battery Bl. When the filament is heated by 

current from B3 electrons are emitted and a certain steady value 

of continuous current flows in the 
plate circuit. If all other factors 

are unchanged this steady value 

depends upon the potential of the 
grid. In Fig. 61 is reproduced 
the characteristic curve of Fig. 
59. We will presume that this 
curve represents the relation be- 
w tween the grid potential and the 

plate current of the tube in the circuit of Fig. 60. 
















60 


THEORY OF RADIO RECEPTION 


303. Now, by means of the potentiometer, the normal po¬ 
tential of the grid may be adjusted so that the plate current pos¬ 
sesses any value represented by some point along the character¬ 
istic curve. 

To operate the tube as a rectifier the grid potential is ad¬ 
justed so that the plate current has a value represented by a 
point on the lower bend of the characteristic curve. This point 
of intersection between the normal grid potential and the normal 
plate current is called the “operating point.” The position of 
the operating point is manifestly adjustable. 

304. Now if a source of alternating e.m.f., Fig. 60, is con¬ 
nected across the grid and filament of the tube, the voltage of the 
grid is alternately raised and lowered above and below normal. 
This evidently results in an alternating increase and decrease of 
the plate current. In the diagram of Fig. 61 are indicated the 
variations of grid voltage caused by the alternations of A and 
the resulting variations of the plate current. The frequency of 
the grid potential and plate current variations, of course, is the 
same as the frequency of A. 



305. It is shown on the diagram that the alternations of A 
raise and lower the potential of the grid by 1 volt above and 
below normal. But on account of the non-linear form of the 
characteristic curve the variations of the plate current above and 
below normal are unequal. The increases are larger than the 
decreases. In other words, equal variations of grid potential 
cause unequal variations of the plate current. 










VACUUM TUBE DETECTOR 


61 


306. If the variations of plate current were equal the average 
value of the current would continue to be the same as the 
normal value; but since the increases are greater than the de¬ 
creases there is an apparent rectification of the plate current 
variations and the average value of the plate current is greater 
than the normal value. 

30 7 . As long as the alternator A impresses an alternating 
e.m.f. across the grid and filament of the tube the plate current 
varies at the frequency of the alternator and the average plate 
current is higher than the normal plate current. 

308. Presuming that the variable factors of the tube circuit 
are adjusted so that the apparent rectification of the plate cur¬ 
rent variations is as efficient as possible for the particular tube 
used, the extent to which the plate current is raised above its 
normal value depends upon the amplitude of the alternating 
e.m.f. across the grid circuit. For instance, if the alternator A 
varies the grid potential above and below normal by 2 volts in¬ 
stead of 1 volt, the average plate current will be still higher 
than before. 

309. An oscillating e.m.f. impressed upon the grid circuit 
of a vacuum tube has a similar effect upon the plate current as 
the alternating e.m.f. of Fig. 60. The plate current varies at 
the high frequency of the impressed oscillations and if the vari¬ 
able factors of the circuit are properly adjusted the plate current 
variations appear to be rectified. The extent to which this in¬ 
creases the average plate current then depends upon the ampli¬ 
tude of the impressed e.m.f. 

310. Receiving circuit with V.T. detector: A possible radio 
receiving circuit employing a vacuum tube as rectifier can there¬ 
fore be represented by the diagram of Fig. 62. The antenna 
circuit can be tuned to res¬ 
onance with the incoming 
signal oscillation by the con¬ 
denser Cl and the secondary 
oscillatory circuit by the 
condenser C2. The two cir¬ 
cuits are coupled by mutual 
induction. Signal oscilla¬ 
tions in the secondary circuit 
create an oscillatory e.m.f. 
across the inductance L2, 
and capacity C2. These e.m. 
the grid and filament of the tube. Any changes in the average 
value of the direct plate current passing through the telephones 
will produce corresponding movements' of the telephone dia¬ 
phragms. High frequency variations of the plate current are 
carried by the condenser C3 which is called a by-pass con¬ 
denser. 

311. If an undamped signal oscillation is impressed on the 
grid circuit the radio frequency variations of the plate current are 
rectified and the average plate current is raised above normal. As 



f. oscillations are impressed between 















62 


THEORY OF RADIO RECEPTION 


the signal oscillation is of constant amplitude, the average plate 
current remains steadily at this higher value as long as the 
signal oscillation is impressed on the grid circuit. 

312. The telephone diaphragms, responding to the changes 
in the average value of the plate current, are pulled closer to 
the magnets by the increase of plate current, but. then remain 
stationary as long as the undamped oscillation is impressed. 
Therefore, this circuit, in common with the crystal rectifier, 
cannot be used to detect undamped waves. 

313. But if the amplitude of the oscillating e.m.f. impressed 
on the grid circuit varies at some audible frequency, as in the 
reception of radio telephony or modulated telegraph signals, the 
average plate current follows the variations in amplitude of the 
incoming oscillations. These audio frequency variations of the 
average plate current vibrate the telephone diaphragms and 
corresponding sound waves are produced. 

314. Spark and I.C.W. telegraph signals can also be de¬ 
tected by the system of Fig. 62. Each wave train increases the 
average plate current. At the end of each wave train the plate 
current returns to normal. As the wave trains follow each other 
at an audible frequency the plate current passing through the 
telephones increases and returns to normal at this frequency. 
Consequently a musical note is produced by the vibrations of 
the telephone diaphragms. 

315. The Operating Point: To detect signals by the method 
of Fig. 62, we have noted the necessity of adjusting the operat¬ 
ing point to the lower bend of the characteristic curve of the 
tube. That is to say, for equal variations of the grid voltage to 
cause unequal variations of the plate current, the normal grid 
potential and the normal plate current must intersect at the 
lower bend of the curve. 

316. Owing to differences of construction all tubes have 
different characteristic curves, even if the variable factors of 
the circuit in which they operate are constant. For this reason 
some tubes are much better detectors than others; some tubes 
are good detectors but cannot be used as amplifiers and so forth. 

317. However, the construction of the tube is not a con¬ 
trollable factor so far as the user is concerned. The manufac¬ 
turer of the tube determines its characteristics; although as 
nearly as possible tubes of each type have approximately the same 
characteristics'. 

318. Methods of Adjusting Operating Point: We have seen 
that the operating point of a tube can be adjusted to ensure the 
best possible rectification by varying the grid potential, as in 
Fig. 62. In this circuit a separate grid battery is used across 
which is. connected a potentiometer resistance. By changing 
the position of the sliding contact of the potentiometer the po¬ 
tential of the grid with respect to the negative end of the fila¬ 
ment can be altered. 

319. However, the operating point can be adjusted without 
necessarily varying the grid potential. It will be recalled that, 
apart from the construction of the tube itself, there are three 


VACUUM TUBE DETECTOR 


63 


factors which determine the characteristic curve of a tube, viz: 
grid potential, filament temperature and plate potential. In 
practice, when a close adjustment of the operating point is re¬ 
quired the plate potential is usually chosen at some approximate 
value which, with a slight variation of either the grid potential 
or the filament temperature, brings the operating point to the 
position of maximum sensitiveness. But a potentiometer is 
required to adjust the grid potential itself and to reduce the 
controls this is often eliminated. The grid potential is then 
fixed and the operating point is found by varying the filament 
temperature, a suitable value of plate potential being chosen for 
the purpose. 


320. When tubes are used in an audio frequency amplify¬ 
ing circuit a close adjustment of the operating point is usually 
unnecessary. The operation of a radio receiver using audio fre¬ 
quency amplification can be greatly simplified by omitting un¬ 
essential controls to closely adjust grid potential or filament 
temperature. The grid potential can be fixed, the filament tem¬ 
perature governed by an automatic control and the plate poten¬ 
tial chosen at some value which will give suitable amplification. 

321. The methods of adjusting the operating point of a tube 
are illustrated in Figs. 63 and 64. In these diagrams the plate 
circuit is not shown. It is understood that the plate potential 
may be varied, if desired, by changing the value of the plate 
battery. However, even when close adjustments are necessary, 
the plate potential is not changed after its proper value is 
chosen. Minute variations, if necessary, are made by altering 
the grid potential or filament temperature. 

322. In Fig. 63 the induc¬ 
tance E is connected across the 
grid and filament of a tube. 

The connection to the filament 
is made directly to the nega¬ 
tive side of the filament battery. 

The normal potential of the 
grid must be the same as that 
part of the filament circuit to 
which it is connected. In 
speaking of grid potential and 
in drawing characteristic curves 
it is customary to compare the potential of the grid with the nega¬ 
tive side of the tube filament. That is to say, if the grid is con¬ 
nected to the negative end of the tube filament it is at “zero po¬ 
tential and since there is a drop of potenial across the filament, 
the grid is then at a negative potential with respect to all other 
parts of the filament. In the same way, if the grid is connected 
to the positive side of the filament it is at a positive potenial in 
relation to the negative and to all other parts of the filament 
except the end to which it is connected. 

323 In the circuit of Fig. 63 the drop across the filament is 
supposed to be 4 volts. A 6 volt battery is used to supply the 
filament current and a rheostat is connected between the filament 















64 


THEORY OF RADIO RECEPTION 


and the negative end of the battery. The drop across the re¬ 
sistance is 2 volts. The grid, being connected to the negative 
side of the filament battery, is therefore at a negative potential 
of 2 volts with respect to the negative end of the tube filament. 

324. With this arrangement the best operating point can be 
found by varying the temperature of the filament with the rheo¬ 
stat R. Incidentally, when the value of R is altered the normal 
potential of the grid in relation to the negative end of the fila¬ 
ment is also changed. 

325. In Fig. 64, a greater range of adjustment of the operat¬ 
ing point is made possible by the use of the potentiometer. In¬ 
stead of connecting the grid directly to the negative side of the 
filament battery as in Fig. 63, the grid is connected to the slid¬ 
ing contact of the potentiometer. If the drop across R is 2 
volts the potential of the grid may be varied from 4 volts posi¬ 
tive to 2 volts negative with respect to the negative end of the 
tube filament. The filament temperature may also be changed 

by the rheostat R. How¬ 
ever, as in Fig. 63, if the 
value of R is altered the nor¬ 
mal grid potential is also 
affected. 

326. If, by moving the 
sliding contact of the poten¬ 
tiometer, the grid becomes 
positive to the negative end 
of the filament, the grid cir¬ 
cuit is conductive. In terms 
of the electron theory, elec¬ 
trons can pass from the filament to the grid or, according to the 
older theory, current can pass from the grid to the filament. As 
a matter of fact the grid circuit may be slightly conductive even 
when the grid is at a negative potential provided the negative 
potential value of the grid is not too great. 

327. The Triode as Detector with Grid Condenser: A more 
efficient use of the three-electrode tube as a detector is by the 
“grid condenser” method represented by the circuit of Fig 65. 
In this case the grid is not connected directly to the filament 
except through a high resistance “leak” of about 1 megohm (1 
million ohms). A small blocking condenser is connected in 
series with the grid. This system of detection may be briefly 
explained as follows: 

328. The grid assumes a 
normal potential which is 
governed by the value of the 
grid leak, the form of the 
grid current curve and the 
potential of the point on the 
filament circuit to which the 
leak is connected. It is best 
to connect the grid leak to 
the positive side of the fila¬ 
ment. 




























VACUUM TUBE DETECTOR 65 


329. At this normal grid potential there is usually a very 
low value of grid current. Owing to the form of the grid cur¬ 
rent curve, if a positive e.m.f. is impressed on the grid which 
raises its potential the grid current is greatly increased. But if 
an equal negative e.m.f. is impressed which lowers the normal 
potential of the grid the grid current is only slightly decreased. 

330. If oscillations are impressed across the input ter¬ 
minals of the circuit of Fig. 65, the grid current varies at the fre¬ 
quency of the impressed oscillation but the grid current varia¬ 
tions are rectified. The increases of grid current are greater 
than the decreases and the average grid current is increased. 

331. The direction of the electrons constituting the grid 
current is from filament to grid. But as there is no easy path 
for the electrons to pass back to the filament the grid accumu¬ 
lates electrons as a result of this increase of average grid cur¬ 
rent. This means that the average potential of the grid is 
lowered below its normal value. 

332. Now as the grid potential varies at the frequency of 
the impressed e.m.f. the plate current also varies at this fre¬ 
quency and as the average grid potential decreases the average 
plate current also decreases. 

333. The extent to which the average grid potential is 
lowered depends upon the amplitude of the impressed oscilla¬ 
tion. If the amplitude is constant the average grid potential is 
lowered to a constant value. If the amplitude increases the 
average grid potential is again lowered. If the amplitude de¬ 
creases the excess electrons on the grid leak off to the filament 
through the high resistance leak and the grid potential is raised. 
If the impressed e.m.f. is removed the grid potential comes back 
to normal. 


334. Therefore, if the impressed oscillating e.m.f. varies in 
amplitude at some audible frequency, as in the reception of radio 
telephony, the average grid potential and average plate current 
vary at this frequency and audible sounds are produced by the 
vibrations of the telephone diaphragms. 

335. Spark and I.C.W. signals can also be detected by this 
system. It is evident, however, that undamped wave signals 
will not be detected. The undamped oscillations impressed on 
the grid and filament reduce the average grid potential and 
average plate current but, as the amplitude does not vary, the 
average plate current remains constant and no vibrations of the 
telephone diaphragms are produced. 

336. It will be noted that 

in the system of detection 
represented by Fig. 62, the 
average plate current is in¬ 
creased by the effect of an 
incoming oscillation whereas 
by the grid condenser sys¬ 
tem the average plate cur-, 
rent is decreased. ' 

337. Fig. 66 shows a ▼ 

complete receiving circuit Flgm 6 

employing the grid condenser method of detection. 












LESSON 6. 


APPLICATIONS OF THE FEED-BACK PRINCIPLE. 

338. The expression “radio frequency amplification” has 
been popularly applied to the multi-stage system of high fre¬ 
quency amplification in which the output of one tube is coupled 
to the input of the succeeding tube through a transformer or 
other repeating device. Before discussing these systems, how¬ 
ever, we must understand the principles of regeneration and the 
use of the vacuum tube as a generator of oscillations. 

339. By a proper arrangement of its circuits a single 
vacuum tube which is used to rectify high frequency oscillations 
can be made to amplify the incoming oscillations themselves. 
In other words, while acting as a rectifier it can act as a radio 
frequency amplifier at the same time, thereby greatly increasing 
the sensitiveness of the receiving system. 

340. The principle which governs the arrangement of the 
circuits to produce this useful effect is called the “feed-back” 
principle and was first defined by Armstrong. 

341. Feed-back System with inductive coupling: Fig. 67 
shows one of the applications of this principle in a radio re¬ 
ceiving system. It will be seen that the only difference between 
this circuit and the circuit of Fig. 66 is the inclusion of the coil 
L3 in the plate circuit. 

342. If this coil is brought close to the coil L2 of the grid 
circuit any current changes in L3 will induce an e.m.f. in L2; 
in other words there will be mutual inductance between the two 
coils and the grid and plate circuits will be inductively coupled. 
Provision is usually made to allow the coil L3 to revolve about 
its own axis so that the coupling between the two circuits can 
be varied. 

343. To study the operation of this circuit let us first pre¬ 
sume that the coupling between the grid and plate circuits is 
zero. In effect the operation of this circuit is then exactly as 
described in connection with Fig. 66. If an oscillating e.m.f. is 
induced by a signal in the circuit L2 C2 the amount of energy 
released in the plate circuit and consequently the loudness of the 
signal in the telephones depends upon the amplitude of the 
e.m.f. oscillations impressed on the grid. The final amplitude 
of the oscillations which build up in the L2 C2 circuit when 


FEED-BACK PRINCIPLE 


67 


tuned to resonance, is limited only by the resistance of this cir¬ 
cuit. No matter how carefully we may arrange the grid circuit 
to avoid unnecessary causes of resistance and consequent losses 
of energy, the circuit will possess some value of resistance which 
will limit the final amplitude of the oscillations. 



344. Now if, in some way, it is possible to overcome the re¬ 
sistance reaction of the grid circuit, the oscillations will build 
up to a greater amplitude than before so that more energy will 
be released in the plate circuit and a louder signal heard. 

345. The resistance reaction of the grid circuit can be par¬ 
tially or completely neutralized by the feed-back system and it 
is in this way that amplification is obtained. 

346. If the coil L3 of Fig. 67 is revolved so that there is a 
slight coupling between the plate and grid circuits the radio fre¬ 
quency current variations in the plate circuit induce an oscillat¬ 
ing e.m.f. in the L2 C2 circuit. The magnitude of this e.m.f. 
depends upon the coupling between the two circuits. 

347. The plate current variations, of course, have the same 
frequency as the signal oscillations in the grid circuit which pro¬ 
duce them. Then if the current through L3 is in the proper di¬ 
rection, the oscillating e.m.f. induced in the grid circuit by the 
current variations of the plate circuit will aid the signal e.m.f. 
and offset the effect of resistance in the grid circuit. 

348. Energy is dissipated by the resistance of the grid cir¬ 
cuit but this loss of energy can be partially or completely coun¬ 
terbalanced by feeding back energy from the plate circuit. The 
amount of energy supplied by the plate circuit and therefore the 
extent to which the resistance reaction is overcome depends 
upon the coupling between the plate and grid circuits. 

349. It is evident that by revolving L3 some value of coup¬ 
ling can be found at which the energy transferred to the grid 
circuit from the plate circuit exactly equals the amount of energy 
consumed by the resistance of the grid circuit. The effective 
resistance of the grid circuit is then zero and the circuit acts as 
though it had no damping effect upon the oscillations. 

350. The value of this simple method of amplification is ap¬ 
parent and it might seem that there is no limit to the magnifica¬ 
tion made possible by its use. There is, however, a decided 
limit. 
















68 


THEORY OF RADIO RECEPTION 


351. Self-generation: If the coupling between the plate and 
grid circuits of the tube is increased beyond a certain value the 
tube becomes a generator of continuous oscillations. It is not 
necessary for a signal e.m.f. to be impressed on the grid circuit 
to produce these self-generated oscillations. They are invari¬ 
ably built up if the coupling is sufficiently close. The slightest 
impulse is sufficient to set up a weak oscillation which gradually 
builds up until it reaches a certain amplitude and then continues 
at this amplitude indefinitely. The impulse may be an atmos¬ 
pheric disturbance, the closing of one of the circuits or an ir¬ 
regularity in electron emission of the tube. No matter how 
slight, it is sufficient to start an oscillation. This oscillation is 
amplified in the plate circuit and fed back to the grid circuit by 
the coupling between the two circuits. The original oscilla¬ 
tion is then reinforced. This releases more energy in the plate 
circuit which is again fed back to the grid circuit. 

352. The amplitude of the oscillation builds up in this man¬ 
ner until sufficient energy is released in the plate circuit to in¬ 
duce an e.m.f. in the grid circuit which maintains the oscilla¬ 
tion against the resistance of the grid circuit. When it reaches 
this amplitude the oscillation continues steadily. 

353. If, while a signal e.m.f. is impressed on the grid cir¬ 
cuit, the feed-back coupling is increased to the point when the 
effective resistance is zero, the conditions for self-oscillation are 
fulfilled. 

354. Regeneration: Now let us see how this feed-back sys¬ 
tem of amplification can be utilized in reception. If the coup¬ 
ling between plate and grid circuits is zero there is no feed¬ 
back amplification. But, if the coil E3 of Fig. 67 is revolved so 
that there is a slight coupling between the circuits the effective 
resistance of the grid circuit is reduced. If a signal e.m.f. is 
induced the oscillations build up to a higher amplitude and a 
louder signal is heard than would otherwise be the case. The 
amplification is said to be obtained by “regeneration.” An oscil¬ 
lating e.m.f. is regenerated in the grid current by the current 
variations in the plate circuit. 

355. If, while a signal e.m.f. is being impressed on the grid 
circuit, the coil L3 is again revolved the amplification increases 
as the coupling between the two circuits is tightened. As the 
resistance of the grid circuit more closely approaches zero, the 
signal becomes louder and louder. 

356. But, if the coupling is increased to the point when the 
energy fed back to the grid circuit from the plate circuit com¬ 
pletely counterbalances the loss of energy due to the resist¬ 
ance of the grid circuit the effective resistance is then zero and 
a more complicated state exists. 

357- A free oscillation is self-generated which continues 
steadily at a constant amplitude irrespective of the signal oscil¬ 
lation. The frequency of this self-generated oscillation is; 


FEED-BACK PRINCIPLE 


69 


governed by the resonant frequency of the grid circuit; the self¬ 
generated oscillation has the frequency to which the grid cir¬ 
cuit is tuned. 

358. If the grid circuit is accurately tuned to resonance 
with the incoming signal oscillation the self-generated oscillation 
and the signal oscillation both have the same frequency. If this 
condition exists the self-generated oscillation very greatly am¬ 
plifies the signal; in practice, however, it is very difficult to so 
accurately tune the grid circuit that the self-generated oscilla¬ 
tion and the incoming signal oscillation have both absolutely 
the same frequency. 

359. If the resonant frequency of the grid circuit is only 
slightly different from that of the signal oscillation the grid 
circuit is then excited by oscillations of two different frequen¬ 
cies. This results in the production of an oscillation of vary¬ 
ing amplitude with a beat frequency equal to the difference in 
frequency between the two separate oscillations. (See Par.104). 
The consequent distortion of signals may not be a very serious 
drawback so far as telegraphic signals are concerned but if the 
modulations of a radio telephone signal are distorted the speech 
or music is unrecognizable when reproduced. 

360. In practice, therefore, it is usually necessary to prevent 
the generation of continuous oscillations in the regenerative sys¬ 
tem and this can only be done by maintaining the resistance of 
the grid circuit at some positive value. The coupling between 
the grid and plate circuits can not be increased beyond the point 
at which the resistance of the grid circuit is almost zero and the 
amplification obtainable by regeneration is thereby limited. 

361. Users of regenerative receivers will realize this fact in 
the operation of the system. Amplification increases as the feed¬ 
back is increased and the greatest amplification is obtained just 
below the point of self-oscillation. If the feed-back coupling is 
still further increased the “tube oscillates” and only distorted 
signals are heard. 

362. Autodyne Reception of Undamped Waves: The fact 
that the vacuum tube can become a generator of continuous os¬ 
cillations is utilized both in radio transmission and reception. 
Vacuum tubes are used as sources of power for the transmission 
of undamped waves, modulated continuous waves (including 
radio telephony) and interrupted continuous waves. A small 
vacuum tube is also used as the generator of the local oscilla¬ 
tions in the reception of undamped waves by the heterodyne or 
“beat” method, as explained in Paragraph 100. The “oscilla¬ 
tor” referred to in that explanation consists of a vacuum tube 
with the plate and grid circuits closely coupled and a variable 
condenser to change the frequency of the oscillations produced. 
An undamped oscillating e.m.f. of any desired frequency is in¬ 
duced in the receiving circuit by this oscillator. 

363. It is possible, however, to use the feed-back system for 
the reception of undamped waves without using a separate 
vacuum tube to generate the local oscillations. For. instance, 
to detect undamped wave signals by the system of Fig. 67 the 


70 


THEORY OF RADIO RECEPTION 


coupling between the grid and plate circuits is increased by 
revolving the “Tickler” coil L3 until a click is heard in the 
phones and the system commences to generate continuous os¬ 
cillations. The frequency of these oscillations can be varied by 
revolving the condenser C2. At any time the frequency of the 
self-generated oscillations is the same as the resonant frequency 
of the grid circuit. 

364. Now, if an undamped signal e.m.f. is impressed on the 
grid circuit the circuit is excited by oscillations of two different 
frequencies. By revolving the condenser C2 the frequency of 
the self-generated oscillations can be varied until the difference 
between the two frequencies produces an audible beat note. 

365. This is called the “autodyne” method of detecting un¬ 
damped waves. It is the same as the “heterodyne” system pre¬ 
viously described with the exception of the fact that only one 
tube is required, the local oscillations being generated by the 
same tube that detects the signals. 

366. There is one important difference between the autodyne 
and heterodyne systems. To detect undamped waves by the 
autodyne method the natural frequency of the grid circuit must, 
of necessity, be different from that of the signal oscillation. 
Therefore, the grid circuit cannot be accurately tuned to the 
incoming signal and the signal oscillations are limited in ampli¬ 
tude by the reactance of the circuit. 

367. This disadvantage is sometimes compensated by the 
fact that only one tube is required; but the heterodyne system 
is invariably more efficient as the grid circuit of the detecting 
tube can be accurately tuned to resonance with the signal fre¬ 
quency, while the frequency of the local oscillations is inde¬ 
pendently varied. 

368. Preferred system for detecting undamped waves: In 

Pig. 68 the heterodyne system of detecting undamped waves is 



used but the detecting circuit is regenerative. Amplification and 
selectivity to a degree which cannot be achieved by the autodyne 
system are easily obtained. The actual arrangement of parts 
suggested by the diagram does not necessarily have to be fol¬ 
lowed but the principle of using a regenerative circuit in con¬ 
nection with an external generator of continuous oscillations is 























FEED-BACK PRINCIPLE 


71 


undoubtedly the most efficient for the reception of undamped 
waves. The advantages of this circuit are particularly apparent 
when long waves are to be detected. 

369. The local oscillations are generated by the “oscillator” 
and their frequency can be varied by means of the condenser C4. 
The antenna circuit and L2 C2 circuit are each tuned to the fre¬ 
quency of the incoming signal oscillation. The coupling be¬ 
tween L3 and L2 is adjusted to bring the resistance of the grid 
circuit to nearly zero. This permits the oscillations in the L2 
C2 circuit produced by the signal to build up to a very high 
amplitude. Being exactly tuned to resonance the total react¬ 
ance is zero while the resistance is very nearly zero, due to the 
feeding back of energy from the plate circuit. 

370. The oscillations generated by the oscillator are intro¬ 
duced into the L2 C2 circuit by the coupling coil L4. The value 
of the oscillating e.m.f. induced in the L2 C2 circuit by the os¬ 
cillations in L4 can be controlled by varying the mutual induc¬ 
tance between L4 and L2. This adjustment has a considerable 
effect upon the strength of a given signal. With a separate os¬ 
cillator it is a simple matter to adjust the amplitude of the local 
oscillation induced in the detecting circuit but it is much more 
difficult to make this adjustment when the autodyne system is 
used. 

The construction and use of an oscillator will be given in 
greater detail in Part 2. 

371. Feed-back system with capacitive coupling: In the 
regenerative circuit of Fig. 67 inductive coupling is used to feed 
back energy from the plate to the grid circuit. In the circuit 
of Fig. 69, however, capacitive coupling is used instead of in¬ 
ductive coupling. 

372. This circuit depends 
for its operation upon the os-W 
cillations which are set up 
across the coil L3 by the radio 
frequency variations of the 
plate current. The coil L3 is 

a variable inductance so that, _ 

with its distributed capacity, it 
can possess any desired reso- 3?- 
nant frequency. 

373. If an oscillating e.m.f. is induced by a signal in the L2 
C2 circuit and the latter tuned to resonance the amplitude of the 
oscillations is limited by the resistance of the grid circuit. The 
plate current r.f. variations have the same frequency as the sig¬ 
nal oscillation. When the plate current increases a self-induced 
voltage is set up across L3 and the direction of this voltage re¬ 
duces the effective e.m.f. of the circuit so that the plate potential 
is lowered below normal. When the plate current decreases a 
self-induced voltage is set up across L3 in the opposite direction, 
increasing the effective e.m.f. of the circuit so that the plate 
potential is raised above normal. 

374. Therefore, as long as the oscillating signal e.m.f. is 
impressed on the grid, oscillations of the same frequency are 



Fig. 69 











72 


THEORY OF RADIO RECEPTION 


produced across the coil L3 in the plate circuit and the plate po¬ 
tential rises and falls above and below normal at this frequency. 

375. If the plate and grid circuits are capacitively coupled 
energy will be fed back from the plate to the grid circuit in the 
proper phase to produce amplification of the oscillations in the 
grid circuit. 

376. The two circuits can be coupled capacitively by means 
of a variable or fixed condenser but the capacity existing between 
the plate and grid inside the tube is sufficient coupling to pro¬ 
duce regeneration if the signal has a high frequency. 

377. Presuming the circuits are coupled through a fixed 
condenser or the capacity of the tube itself the amplification ob¬ 
tained depends upon the extent to which the plate potential rises 
and falls. This, in turn, depends upon the amplitude of the 
oscillations across E3. The latter can be increased by varying 
the inductance of L3 so that, with its distributed capacity, its 
resonant frequency approaches the frequency of the oscillations 
produced across it by the r.f. plate current variations. 

378. If E3 is tuned to exact resonance and the frequency is 
high, the energy fed back from the plate circuit to the grid cir¬ 
cuit will be sufficient to sustain continuous oscillations in the 
system so that the tube becomes a generator of oscillations. If 
the frequency is low or if the internal capacity of the tube is 
very small, continuous oscillations can be generated by increasing 
the capacitive coupling between plate and grid circuits with an 
external condenser. 

379. The system of Fig. 69 is greatly used today for the re¬ 
ception of short wave signals—undamped waves by the auto¬ 
dyne system and others with regenerative amplification. Some¬ 
times a variometer is used to continuously vary the inductance 
of E3 and at others a variable condenser is connected across E3 
to tune the plate oscillatory circuit. The principle of operation 
is the same in every case. 



Fig. 70 


380. Fig. 70 shows the 
most popular application of 
this principle with internal ca¬ 
pacitive feed-back coupling. 
)A variometer is used in place 
of a variable condenser to tune 
the grid circuit to resonance. 
The object of this is to tune 
the grid circuit to resonance 


with a preponderance of inductance so that the e.m.f. oscilla¬ 
tions will have as great an amplitude as possible. The advantage 
of this is sometimes lost in the distributed capacity of the vario¬ 
meter. 


381. Single-circuit vs. Loose-coupled Regenerative Receiv¬ 
er : One of the principal advantages of the regenerative receiver 
is the selectivity it affords. The regenerative action reduces the 
effective resistance of the grid circuit to some value closely ap¬ 
proaching zero. This not only increases the amplitude of the 
oscillations in the grid circuit but makes the circuit very 
selective. 












FEED-BACK PRINCIPLE 


73 


382. There is frequently some debate amongst amateurs 
as to whether a single circuit” or an inductively coupled re¬ 
generative receiver is the more desirable. Both are selective 
and sensitive but there is no question whatsoever that the loose- 
coupled receiver permits a very much higher degree of selectivity 
and also allows a finer control of regeneration than the other. 
These advantages, in our opinion, outweigh the merit of sim¬ 
plicity possessed by the single circuit receiver. Figure 71 shows 
the latter circuit. LI is the primary of a vario-coupler (with a 
fairly close coupling) and L2 
the secondary which is used as 
the tickler coil; the antenna cir¬ 
cuit can be tuned to resonance 
with the variable condenser Cl. 

This circuit is only selective 
because the regenerative action 
reduces the effective resistance 
of the antenna circuit. The 
audibility is good and a fair 
degree of selectivity is obtained but regeneration is somewhat 
difficult to control. 

383. With the inductively-coupled regenerative circuits 
shown in Figs. 69 and 70 both the antenna circuit and the second¬ 
ary circuit are tuned to resonance. The antenna, because of its 
size and resistance cannot be tuned to any particular frequency 
without being subject to forced oscillations of other frequencies. 
Static and atmospheric strays of varying frequencies all pro¬ 
duce forced oscillations in the antenna circuit, irrespective of 
the wave-length to which it is tuned, although oscillations of the 
latter frequency persist longer and reach a greater amplitude 
than the others. The secondary circuit, however, can be accu¬ 
rately tuned to any desired frequency and is subject to forced os¬ 
cillations of other frequencies only to a very slight extent. If, 
by regenerative action, the resistance of the secondary circuit 
approaches zero, it is extremely selective. 

384. In discussing coupled oscillatory circuits in Lesson 4, 
we observed the necessity of maintaining a loose-coupling if 
selectivity is to be gained. An inductively coupled regenerative 
receiver, however, necessitates a looser coupling than the non- 
regenerative circuit because the energy in the secondary circuit 
is amplified by regeneration. As the energy is increased the 
reaction between the secondary circuit and the antenna circuit 
is also increased and, to maintain resonance, it is necessary to 
loosen the coupling. This, of course, automatically results in 
an increase of selectivity, as the looser coupling prevents to 
a greater extent the transfer to the secondary circuit of forced 
oscillations of undesired frequencies in the antenna circuit. 

385. Another desirable feature of the loose-coupled receiver 
is the fact that less energy is lost by re-radiation from the an¬ 
tenna. The single circuit regenerative receiver radiates strongly. 
This is undesirable, from the standpoint of conserving energy 
and also because of the interference which this re-radiation 
causes to other receiving stations in the neighborhood. 

















LESSON 7. 


RADIO AND AUDIO FREQUENCY AMPLIFIERS. 


386. The Triode as an Amplifier: It will be realized that the 
three-electrode vacuum tube is, in effect, an electrical relay. It 
is impossible to magnify energy but, by a relay system, we can 
make a small amount of energy control a large amount of energy. 
This is what takes place in the vacuum tube. A small variation 
of the potential of the grid produces a large variation of the 
plate current. But the grid circuit and the plate circuit are 
separate and distinct. The energy in the plate circuit is not 
supplied by the incoming signal voltage. The latter, acting upon 
the grid, merely releases energy in the plate circuit supplied by 
the plate battery. 

387. By this relaying system, a small amount of energy ex¬ 
pended on the grid circuit releases a large amount of energy in 
the plate circuit. The amplified energy which is released in the 
plate circuit is utilized instead of the feebler energy impressed 
on the grid circuit. 

388. The amplification which can be obtained with a single 
tube is, of course, limited; but just as one relay may be used to 
operate a second relay or any number of relays in succession— 
each relay multiplying the energy of its predecessor until finally 
a great amount of energy is controlled by a small initial force— 
so vacuum tubes may be used to amplify the permutations of 
electric current. 

389. An “amplifier,” then, usually consists of two or more 
vacuum tubes arranged so that a varying voltage impressed on 
the grid of the first tube will produce a similar and undistored 
variation of the plate current; this variation of the plate cur¬ 
rent must then produce a varying voltage between the grid and 
filament of the second tube. A transformer or other device is 
used to repeat the amplified variations from one tube to the next. 
In a similar way the variations are relayed from tube to tube. 

390. The diagram of Fig. 72 shows how the amplifying tube, 
without causing distortion, relays the variations impressed on 
the grid. This diagram shows the characteristic curve of an 
amplifying tube. The variations of grid potential and corres¬ 
ponding variations of plate current are indicated. As the operat¬ 
ing point is chosen on the straight portion of the characteristic 
curve, equal variations of grid potential produce equal varia- 


AMPLIFIERS 


75 


tions of plate current. Even if the variations of grid potential 
have the complicated form of a speech wave they will be exactly 



reproduced in the plate circuit. There will be no distortion, 
provided the operating point is correctly chosen. 

391. Functions of Radio and Audio Frequency Amplifiers: 

Amplifiers, as used to aid radio reception, are divided into two 
main divisions: 

1. Radio Frequency Amplifiers. 

2. Audio Frequency Amplifiers. 

Basically the two types of amplifiers operate under the same 
principles. In each case the amplifying characteristics of 
vacuum tubes are utilized to magnify varying voltages and cur¬ 
rents without distorting their original form. The audio fre¬ 
quency amplifier magnifies variations of a low audible frequency 
whereas the radio frequency amplifier magnifies the very high 
frequencies used in radio transmission. For this reason the 
amplifiers of the two systems are not interchangeable. Of ne¬ 
cessity the apparatus in the circuits is designed according to the 
frequency of the currents to be amplified. 

392. The exact function of each division of a radio receiver 
employing both radio and audio frequency amplification is briefly 
recapitulated below and illustrated by the drawing of Fig. 73, 










76 


THEORY OF RADIO RECEPTION 


the latter showing the logical sequence of the various divisions. 

393. Tuner or Receptor: Radio waves induce in the receiv¬ 
ing antenna an oscillating e.m.f. The frequency of the oscil¬ 
lations depends upon the frequency of the waves or, in radio par¬ 
lance, upon the wave-length. The frequency is always high- 
above audibility. The strength or amplitude of the signal e.m.f. 
depends upon the distance between the transmitter and the re¬ 
ceiver, the design of the receiving antenna, the power of the 
transmitter and other variable conditions outlined in Paragraph 
86. The signal oscillations may be of constant amplitude or 
varying amplitude, continuous or in trains, according to the type 
of transmission producing the waves. 



The function of the tuner is to adjust the receiving circuit 
or circuits to resonance with the particular signal oscillation to 
be detected, thereby permitting current and voltage oscillations 
of the greatest possible amplitude to build up in these circuits 
and offering impedance to oscillations of different frequencies 
produced by other waves. The lower the resistance of the tun¬ 
ing circuits the higher the amplitude of the resonant oscillations 
and the greater the selectivity of the receiver. 

394. Radio Frequency Amplifier: This amplifier is used to 
magnify the high frequency oscillations of incoming signals 
before rectification. The oscillations in the tuner circuits may 
be too weak to operate the rectifier. By radio frequency ampli¬ 
fication they are magnified without distortion and the oscilla¬ 
tions which are finally rectified have a very much greater ampli¬ 
tude than the oscillations in the tuner circuits. 

395. Detector: The high frequency oscillations are rectified 
by the detecting system. The strength of the rectified current 
varies in step with the variations in amplitude of the impressed 
high frequency oscillations. As these variations of amplitude 
occur at an audible frequency the varying current produced by 
the rectifying action will cause audible vibrations of a telephone 
receiver diaphragm if the current flows through the coils of the 
telephone. The rectifier with the telephone detects the signal 
—makes it audible. 

396. Audio Frequency Amplifier: This amplifier increases 
the audibility or loudness of the detected signal. The varia¬ 
tions magnified by audio frequency amplification are the varia¬ 
tions in strength of the rectified current produced by the de¬ 
tecting system. 



















AMPLIFIERS 


77 


397. Reproducer: The signals are finally reproduced by the 
sohnd waves created by the vibrations of a telephone diaphragm. 
The sound waves themselves may be amplified and the quality of 
reproduction improved by means of a horn or other acoustical 
amplifier. 

398. It should be quite clear, that whereas the audio fre¬ 
quency amplifier increases the audibility of a detected signal, 
the radio frequency amplifier makes possible the detection of 
signals which could not otherwise be heard; it increases the 
sensitiveness of the receiver. 

399. Just what is meant by this may be understood better 
if we say that radio frequency amplification, in effect, brings 
the transmitting station closer to the receiving station. For 
instance, if a certain amateur receiving station A is 1000 miles 
away from a transmitting station B and A is tuned accurately to 
the wave-length on which B is transmitting, the oscillations in 
the receiving circuits of A may be too feeble to operate the 
rectifier. The signals of B are inaudible. Now A can increase 
the amplitude of the oscillations and make the signals from B 
audible by moving closer to transmitter B but he can achieve the 
same result by using a radio frequency amplifier to magnify the 
oscillations. 

400. To give another example of the use of a radio frequency 
amplifier: Suppose that it is inconvenient for the owner of a 
receiver to erect an aerial and instead he uses a small indoor 
antenna. His receiving range is naturally limited. However, if 
he uses a radio frequency amplifier the sensitiveness of his ap¬ 
paratus will be greatly increased and his receiving range ex¬ 
tended. 

401. Classification of Amplifiers: Amplifiers, whether for 
radio or audio frequency amplification, are divided into three 
main classes: 

1. Resistance—Coupled Amplifiers. 

2. Inductance—Coupled Amplifiers. 

3. Transformer—Coupled Amplifiers. 

These classifications are made according to the arrangement 
used for repeating the variations from one tube to the next. 

RESISTANCE—COUPLED AMPLIFIERS. 

402. Fig. 74 illustrates the principles of a resistar.ee- 
coupled amplifier. The varying voltage to be amplified, at radio 
or audio frequency as the case may be, is applied across the 
input terminals which connect to the grid and filament of the 
first tube. The object of tube A is then to produce a vary¬ 
ing voltage across the points X and Y of its external plate cir¬ 
cuit. This voltage must be similar in form to that impressed 
on the grid and as much larger as possible. The amplified volt¬ 
age variations are then applied between the grid and filament 
of tube B, as shown. The capacity Cl is inserted to prevent the 
steady voltage of B2 from affecting the grid of the second tube. 
Only the variations in voltage are impressed on the grid through 
the capacity Cl. 


78 


THEORY OF RADIO RECEPTION 



Fig. 74 


403. Let us see how variations of voltage are produced 
across the points X and Y of tube B. We know that a change 
of grid potential changes the plate current of a tube. But if 
any resistance is connected in the plate circuit, outside the tube, 

a variation of grid potential 
changes the potential drop 
cross this resistance. 

404. In Fig 74, the com¬ 
plete plate circuit of tube A is 
formed by the filament and the 
plate (within the tube) the re¬ 
sistance R1 and the plate bat¬ 
tery B1 (external). The plate 
circuit, of course, offers impedance to the current variations which 
flow in it as a result of the grid potential variations. The plate- 
filament (internal) impedance of the tube is usually determined 
mainly by the resistance of this path which, for the tubes used 
in radio receivers, may be in the neighborhood of 5,000 to 30,000 
ohms. Similarly the impedance of R1 is usually determined by 
its resistance alone. It is constructed to possess no inductance 
and as small a capacity as possible. 

405. It should be noted, however, that both the plate-fila¬ 
ment path and the external resistance R1 have capacity as well 
as resistance. The values of these capacities are very small 
and have little or no effect upon the impedance of the plate cir¬ 
cuit when the current variations are of low frequency. But, the 
reactance of a given capacity decreases with an increase of fre¬ 
quency and if the current variations in the plate circuit have a 
high frequency the capacity reactances of the tube and the re¬ 
sistance R1 may be low enough to determine the impedance of 
the plate circuit; if the frequency is high enough the impedance 
may be less than the resistance. 

406. When the grid potential of tube A is changed, the 
plate current is changed. But the current in a circuit can only 
be changed by a change in the e.m.f. or the resistance. Since 
the plate current is changed by a variation of grid voltage with¬ 
out altering the e.m.f. of the plate battery, it is evident that the 
change of grid voltage alters the plate-filament resistance of 
the tube. 

407. The complete plate circuit of 
tube A, therefore, can be represented by 
the simple electrical circuit of Fig. 75. 

B1 represents the plate battery, R1 the 
external plate circuit resistance and R2 
the variable resistance of the tube. The 
resistance of R2 depends upon the voltage 
of the grid. If a varying e.m.f. is impressed 1 —tH• I* I• 111 

on the grid the resistance R2 varies 3 , Fig. 75 

at the frequency of the impressed e.m.f. The resistance of R1 is 
constant. The total resistance of the plate circuit is equal to R1 
plus R2. The current in the circuit, by Ohm’s Law, is equal to 
the e.m.f. of B1 divided by the total resistance of the circuit. 
The drop in potential across R1 at any moment is equal to the 













AMPLIFIERS 


79 


current in the circuit at that moment, multiplied by the resistance 
of Rl. It is evident, therefore, that if the current through the 
constant resistance Rl is varied by a variation of R2, the poten¬ 
tial drop across Rl also varies. 

408. Referring again to Fig. 74, then, the variations of the 
grid potential of tube A produce a varying difference of po¬ 
tential across the high resistance Rl and since the resistance 
of battery B1 is low, it follows that a varying difference of po¬ 
tential of almost the same value is produced across the points 
X and Y. 

409. Voltage Amplification of Resistance Amplifier: The 

voltage amplification obtained by tube A is determined by the 
extent to which a given variation of grid voltage changes the 
potential across the external plate circuit. For instance, if a 
change of grid potential of 1 volt produces a change in the po¬ 
tential across the external plate circuit of 10 volts, the voltage 
amplification is 10. 

410. The voltage amplification can be increased by increas¬ 
ing the resistance of Rl, at the same time increasing the volt¬ 
age of Bl, but it is impractical to increase the voltage of B1 
indefinitely and the gain in am¬ 
plification after Rl passes a 
certain value of resistance is 
very small, even for large in¬ 
creases of Rl. The useful value of Rl depends upon the type 
of tubes employed and is usually from 20,000 to 100,000 ohms. 
Fig 76 shows a photograph of a lavite resistance which is suit¬ 
able for use in a resistance-repeating amplifier. This resistance 
is non-inductive and has very small capacity so that its impedance 
is practically constant for all frequencies. 

411. Repeating Action: The amplified variations of voltage 
produced across the points X Y of Fig. 74 are applied between 
the grid and filament of tube B. The amplified signal is then 
reproduced by the telephones in the plate circuit of this tube. 
The external plate circuit (XY) across which the voltage varia¬ 
tions are produced is shunted by the grid-filament circuit of tube 
B. This circuit is formed by the leak R2 and the capacity and 
resistance of grid to filament within tube B. Provided that the 
impedance of this circuit is much greater than the resistance of 
Rl it will not affect the voltage across X Y. But if the imped¬ 
ance of the grid-filament circuit of tube B is less than the im¬ 
pedance of Rl, it will very seriously reduce the voltage across 
X Y. The amplification will then be diminished. 

412. Grid Condenser and Leak: The value of the grid con¬ 
denser Cl depends upon the frequency of the current variations 
to be amplified by the system although this condenser does not 
require a close adjustment. The reactance of the condenser, 
of course, depends upon its capacity and the frequency of the 
variations which it carries. If its reactance is too high, as com¬ 
pared with the reactance of the grid to filament circuit, the 
drop in potential across the condenser will reduce the voltage 
impressed on the grid. 

413. On the other hand, if the capacity of the grid condenser 



80 


THEORY OF RADIO RECEPTION 


is increased to reduce its reactance, the capacity may be so large 
that the amplifier will be “paralyzed.” The condenser will take 
too long to discharge. If a sudden pulse of e.m.f. is impressed 
on the condenser the grid may become so negative that the plate 
current is reduced to zero. If the condenser cannot quickly dis¬ 
charge, the amplifier will be “dead.” Of course, this could be 
remedied by reducing the value of the grid leak R2, but if R2 
becomes too low it reduces the impedance of the grid to fila¬ 
ment circuit; this will tend to reduce the voltage across X Y and 
diminish the amplification. 

414. The values of the grid condenser and grid leak, there¬ 
fore, are chosen to obtain the maximum possible efficiency ac¬ 
cording to the frequency of the variations to be amplified. 
Roughly, the grid condenser should be about .0005 mfd. for fre¬ 
quencies above 100,000 cycles per second (wave-length 3,000 
meters), and about .003 mfd. for audio frequencies. The grid 
leak should be from one to five million ohms. The best value is 
found by experiment. 

415. Resistance Amplifier for Audio Frequencies: This type 
of amplifier for audio frequency amplification is practically obso¬ 
lete. One of its disadvantages is that the resistances absorb a 
large part of the voltage of the plate battery. The plate battery 
voltage must be made from two to four times the normal value. 
In any case the amplification per stage is much less than with 
the transformer-coupled amplifier. 

416. Resistance Amplifier for Radio Frequencies: Fig. 77 
shows the connections of a one-stage resistance amplifier and 
detector. This amplifier is intended, of course, to magnify radio 
frequency signal oscillations. 

The sole advantage of the 
resistance-coupled r.f. amplifier 
is that it magnifies signals over 
a very wide range of frequen¬ 
cies. It will amplify signals 
from 1,000 meters up to, say 
15,000 meters. This feature 
makes it rather useful for com¬ 
mercial work and on shipboard 
ig. 77 ©***, **u> £ or reception of long-wave 

. . signals over a wide range of 

frequencies. 

f.]/" However, the resistance amplifier is useless for the 
amplification of short-wave signals and the reasons for this 
should be understood, as the same difficulties are invariably en¬ 
countered in designing a radio frequency amplifier for short 
waves. 

418. The voltage amplification obtainable with a short-wave 
radio frequency amplifier of any type is never as great as that 
which can be obtained with a long-wave amplifier or an audio 
frequency amplifier. This does not mean that the amplification 
of short waves is impractical. It merely means that the ap¬ 
paratus employed must be very carefully designed and the am¬ 
plifier constructed to ensure the maximum possible amplification 

















AMPLIFIERS 


81 


419. The great obstacle to the amplification of short waves 
is the low reactance of even small capacities in the circuits to 
these high frequency currents. For instance, when a resistance 
amplifier is used for audio frequency amplification, the reactance 
of the grid to filament capacity of a tube is from one to two 
million ohms so that the impedance of the grid filament cir¬ 
cuit is entirely determined by the actual resistance of grid to 
filament (together with the resistance of the grid leak). This 
resistance may be about 200,000 ohms. But if the amplifier is 
used for high frequencies the reactance of the grid to filament 
capacity may be as low as five or six thousand ohms. The im¬ 
pedance of the grid-filament circuit is then almost entirely de¬ 
termined by this reactance which is considerably less than the 
actual resistance of the tube. The same conditions apply in 
the plate-filament circuit. The reactance is high for low fre¬ 
quency currents but is quite low for high frequency currents. 
Additionally any long wires connecting the apparatus together, 
the tube bases and sockets and other apparatus in the circuits, 
may possess capacity which will reduce the impedance of the 
plate-filament and grid-filament circuits. 

420. Referring back to. Fig. 74 it will be evident that if 
the resistance Rl, which may have a value of 50,000 to 100,000 
ohms, is shunted by a grid-filament impedance of 6,000 ohms, the 
impedance of the external plate circuit is so greatly reduced that 
there is very little voltage amplification. If a resistance am¬ 
plifier with four or five tubes is used to magnify the high fre¬ 
quency oscillations of a 200-meter wave it is conceivable that the 
voltage finally impressed on the last tube may be even less than 
the voltage impressed on the first tube. The amplifier is then 
useless. 

421. The resistance amplifier can be used for wave-lengths 
down to about 600 meters if special tubes are employed with a 
very low internal capacity—particularly between grid and fila¬ 
ment. The Meyers tube has the least capacity of any tube at 
present obtainable and would be especially suitable for use in 
a resistance amplifier. In constructing an amplifier of this type 
it is very necessary to use the shortest possible leads between 
the apparatus in the circuit, particularly the grid and plate leads, 
to eliminate capacity effects. 

INDUCTANCE-COUPLED AMPLIFIERS. 

422. An inductance-coupled amplifier is similar to the resist¬ 
ance-coupled amplifier except that an inductance is used in the 
plate circuit of each amplifying tube instead of a resistance. In¬ 
stead of relying on the resistance of the external plate circuit 
to produce potential variations across it when the grid voltage 
is varied, the same effect is obtained by the reactance of an in¬ 
ductance with a comparatively low D.C. resistance. 

423. Fig. 78 illustrates the principles of an inductance- 
coupled amplifier. It will be seen that this corresponds exactly 
with the diagram of Fig. 74 except that an inductance is used 
instead of a resistance in the external plate circuit. Again the 


82 


THEORY OF RADIO RECEPTION 


object of tube A is to produce a varying voltage across the points 
X and Y, this voltage to be similar in form to that impressed on 
the grid and as much larger as possible. 

424. The manner in which this is accomplished should be 
fairly evident. When the grid potential of tube A is varied, the 
plate current varies. But the self-induction of L causes a re- 

_ acting voltage to be set up 

Tube0 I across the coil, acting in oppo- 
N ^sition to the changes of current 
' Y passing through it. Therefore 

1 variations of the grid potential 
-j of the tube A produce a vary- 
r 1 ing voltage across the points 

F&. 78 X and Y of the external plate 

circuit. The manner in which these variations are repeated from 
tube to tube correspond exactly with the repeating action of the 
resistance, amplifier. An advantage of the inductance amplifier, 
however, is that the coil offers very little resistance to the flow of 
direct current in the plate circuit so that the plate battery can 
be of normal value. 

425. The voltage amplification of an inductance-coupled am¬ 
plifier depends upon the reactance of the inductance. The great¬ 
est amplification is obtained when the reactance is infinitely high. 
But the gain in amplification after the reactance exceeds a cer¬ 
tain value is very small. Therefore the repeating inductance 
is designed to have a certain high reactance depending upon the 
tubes employed. The actual inductance of the coil (that is to 
say, the number of turns, etc.) which gives this reactance, de¬ 
pends upon the frequency for which the amplifier is designed. 
For instance, suppose it is found that a reactance of 30,000 ohms 
gives good amplification with the tubes to be used in the am¬ 
plifier, and the coils are wound to possess an inductance which 
will offer this reactance to an audio frequency of say 1,000 cycles 
per second. The coils would have such a comparatively high 
distributed capacity that the capacity reactance to a high fre¬ 
quency current would be very low. High frequency currents 
would be carried by the capacity of the coil and hardly any 
reacting voltage would be set up. 

The coils of the inductance-coupled amplifier, therefore, must 
be designed according to the frequency to be amplified. 

426. Inductance-Coupled Amplifier for Audio Frequencies: 
The inductances for this type of amplifier are wound on an iron 
core and constructed to possess a low value of distributed ca¬ 
pacity. The amplifier operates over a comparatively wide range 
of low frequencies and gives fairly even amplification over this 
band. Unless the frequency is above the upper limit of the 
range for which the amplifier is designed, the impedance of 
the inductances is not affected to any extent by their distributed 
capacity. The inductance-coupled amplifier for audio frequencies, 
however, is obsolete as much greater amplification can be ob¬ 
tained with the transformer-coupled system. 

_ 4 27- Inductance-Coupled Amplifier for Radio Frequencies* 
The effects of distributed capacity prohibit the construction of a 














AMPLIFIERS 


83 


high frequency amplifier which will operate over a broad band 
of frequencies. Suppose that a reactance of 30,000 ohms is the 
most suitable value for the coils in the amplifier. If, without 
considering the effect of distributed capacity, an ordinary solen- 
oidal coil is wound to give an inductance reactance of 30,000 
ohms to a frequency of, say 1,000,000 cycles (300 meters), the 
actual reactance for this frequency, due to the capacity of the 
coil, is much less than 30,000 ohms. Small capacities, which 
hardly affect the impedance of the coils in the audio frequency 
amplifier, have a serious effect upon the impedance of the coils 
in the radio frequency amplifier. 

428. The effect of the distributed capacity of a coil in the 
radio frequency amplifier is to make the coil, with its capacity, 
a tuned circuit with a resonant frequency within the range of 
frequencies covered by the amplifier. A signal at this resonant 
frequency is amplified well—since the oscillations set up across 
the tuned circuit formed by the coil and its distributed capacity 
are a maximum at this frequency. But the amplification of other 
signals of different frequency is poor. 

429. This undesirable feature of the inductance-coupled high 
frequency amplifier can be overcome by actually tuning the.ex¬ 
ternal plate circuit of each tube in the amplifier. If the amplifier 
is intended for the amplification of very short waves—say 200 
to 500 meters—the plate circuits can be tuned to resonance with 
the signal frequency by means of variometers; or the circuits can 
be tuned by connecting a variable condenser across each plate 
inductance. In this way the maximum possible amplification 
can be obtained for any wave-length within the range covered 
by the tuning of the circuits. 

430. This “tuned impedance” 
system of r.f. amplification is quite 
efficient but not so efficient as the 
transformer-repeating systems we 
shall presently describe. How¬ 
ever, before passing on the third 
class of amplifier, we wish to call 
attention to the circuit of Fig. 

79, representing a tuned impedance high frequency am¬ 
plifier which will be found of practical service to many. While 
we do not believe it to be as efficient as the transformer-coupled 
system, we show this circuit because many amateurs will be able 
to adopt it with only a slight rearrangement of their present 
apparatus and a few inexpensive additions. 

431. The circuit shows a simple method of adding one stage 
of radio frequency amplification to a standard regenerative re¬ 
ceiver with tuned plate circuit. The addition requires only one 
tube a few accessories and simple changes in the wiring of the 
receiver. This can easily be verified by comparing the circuit 
of Fig 79 with the simple regenerative circuit of Fig. 70. The 
number of controls is not altered but the sensitiveness of the 
system is greatly increased. The addition does not change the 
operation of the receiver in any way. The grid circuit is tuned 
with the grid variometer and the plate circuit with the plate 












84 


THEORY OF RADIO RECEPTION 


variometer. When the plate circuit is tuned to resonance, con¬ 
tinuous oscillations are generated and C.W. signals detected. 
With the plate circuit tuned just below the point of self-oscil¬ 
lation, high amplification is obtained of spark and radiophone 
signals. Chapter 8 gives particulars of the type of vario-coupler 
which should be used with this circuit. 

TRANSFORMER-COUPLED AMPLIFIERS. 



432. Transformer-coupled amplifiers have almost entirely 
superseded the resistance-coupled and inductance-coupled types, 
They have many advantages over the other systems. 

433. Audio Frequency Amplifiers: The great advantage of 
this type of amplifier is the step-up of voltage which can be ef¬ 
fected by means of the transformers. The varying voltage im¬ 
pressed on the grid of each tube 

-fiP-nr 51 -i,_r-n r- -iof the amplifier may be four 

9 or five times as great as the 
voltage across the external plate 
= 11 * 1 * 1*1 circuit of the preceding tube. 
Fig. 80 . 434 . Fig. 80 shows the 

principles of a receiver with one stage of transformer-coupled 
audio frequency amplification. The primary of the trans¬ 
former Tr. is connected in the plate circuit of the detect¬ 
ing tube. The secondary of the transformer is connected 
across the grid and filament of the amplifying tube. The 
transformer is wound with a step-up ratio of turns, and is de¬ 
signed so that any audio frequency changes of current in the pri¬ 
mary induce a high voltage in the secondary. Thus, when a sig¬ 
nal oscillation is impressed on the grid circuit of the detector 
tube, the radio frequency variations of the plate current are car¬ 
ried by the by-pass condenser while the audio frequency changes 
in the average plate current produce variations in the intensity 
and direction of the magnetic field round the primary of the 
transformer. An alternating e.m.f. of similar form is induced 
in the transformer secondary and applied between the grid and 
filament of the amplifying tube. As the telephones are included 
in the plate circuit of the second tube, the received signals are 
reproduced with much greater volume of sound. 

435. For the sake of clarity Fig. 80 shows independent fila¬ 

ment and plate batteries for each tube. However, this is un¬ 
necessary in practice. The same batteries can be used for the 
two tubes and Fig. 81 shows the 
connections. In operation, the fj j l °^ cfor 

circuit of Fig. 81 is exactly the 
same as that of Fig. 80. 

436. If still louder reproduc¬ 
tion of signals is desired, a two 



or three-stage audio frequency amplifier can be used to mag¬ 
nify the output of the detector tube. With more than three 
stages unpleasantly loud signals are produced and unless there 
is some special need for terrific volume of sound, three stages 
are sufficient. Fig. 82 shows the wiring diagram of a two-stagr 






















AMPLIFIERS 


85 


amplifier. The “input” terminals are connected in the plate cir¬ 
cuit of the detector tube. 

437. In these diagrams it should be noted that the grid of 
each amplifying tube is connected through the secondary of the 
transformer to a point of negative potential on the filament. The 
reason for this is that the impedance of the grid-filament circuit 
must be held at a high value. It is 
necesary to maintain the grid at such 
a negative potential with respect to 
the filament that even a strong signal 

will not raise the grid potential _ 

above zero. If the grid becomes Fig . 82 

positive with respect to the filament, the grid-filament circuit 
is conductive—a grid current can flow. This reduces the volt¬ 
age across the secondary of the transformer. Power is consumed 
in the production of a grid current. In fact, if the normal poten¬ 
tial of the grid is positive in relation to the filament there may be 
practically no amplification. The voltage across the secondary 
of the transformer may.be less than the voltage impressed across 
the primary. 

438. To avoid grid current losses the grid must be connected 
through the secondary of the transformer to a point on the fila¬ 
ment circuit which is about 1 or 1 ^4 volts negative with respect 
to the negative end of the filament. The method of accomplish¬ 
ing this is shown in the diagrams. This was also illustrated in 
Fig. 63 and described in Paragraph 322. 

439. The Audio Frequency Transformer: The efficiency of 
an a.f. amplifier chiefly depends upon the transformers employed. 
If the plate and filament batteries have the correct values for 
the tubes used and the apparatus is connected together properly, 
good amplification should easily be obtained unless the trans¬ 
formers are poorly designed or constructed. The object of the 
transformer, of course, is to make the audio frequency current 
variations in the plate circuit of one tube induce the highest 
possible e.m.f. in the secondary which is connected across the 
grid and filament of the succeeding tube. 

440. We can only touch briefly upon the many factors which 
enter into the design of the audio frequency transformer. The 
coupling between the primary and secondary should be as close 
as possible so that the e.m.f. induced in the secondary by a given 
current change in the primary may be as high as possible. 

441. It is, in practice, impossible to obtain 100 per cent 
coupling between two circuits, no matter how the transformer is 
designed. The mutual induction can never be as great as the 
square root of the product of the self-induction of each circuit. 
Some of the magnetic field of one circuit does not interlink with 
the other circuit through the transformer. There is some magne¬ 
tic leakage. 

442. To obtain as close coupling as possible and also be¬ 
cause of the necessary high inductance of both primary and sec¬ 
ondary windings the transformer is wound on a closed iron core, 
the secondary being wound over the primary, as illustrated in 
Fig. 83. 










86 THEORY OF RADIO RECEPTION 

443 . The close coupling this affords can be explained as 
follows: Owing to the greater permeability of iron, any mag¬ 
netic field set up by an increase of current in the primary coil 
builds up from the primary windings and takes the position in¬ 
dicated in the iron core. Very little of the magnetic flux remains 
round the primary coil itself. However, before taking up its 
position in the iron core, the magnetic field must expand through 
the air space between the primary coil and the iron and in so 

doing it cuts through the secondary 
windings which are laid over the pri¬ 
mary! Similarly if the current in the 
primary decreases, the magnetic flux 
contracts and leaving the iron core, it 
cuts through the secondary windings 
as it collapses upon the primary. Thus 
very nearly all the magnetic flux set 
up by the primary interlinks with the 
secondary and the coupling is very 
close. The iron must be of finest quality and carefully 
laminated to reduce iron losses to a minimum. 

444. To further increase the value of the e.m.f. induced 
in the secondary, the secondary is wound with more turns than 
the primary. The voltage is stepped up in the ratio of the num¬ 
ber of turns in the primary to the number of turns in the sec¬ 
ondary. 

445. The impedance of the primary has to be chosen in 
accordance with the frequency of the currents to be amplified 
and the type of tubes employed. To increase the ratio of trans¬ 
formation, the impedance of the secondary should be high; but, 
on the other hand, if the secondary is wound with too many 
turns the resulting increase of distributed capacity may reduce 
the impedance and the voltage amplification. It has been found 
that the transformer with a ratio of 4 or 5 generally gives a 
greater step-up of voltage than one with a higher ratio. 

446. The efficiency of a transformer, however, is not alone 
judged by the voltage amplification it affords. It must be de¬ 
signed to amplify uniformly over a fairly wide range of fre¬ 
quencies. This is especially important if the transformer is to 
be used in the audio frequency amplifier of a radiophone re¬ 
ceiver. Speech and music cover all the frequencies from about 
100 to 4,000 cycles. If the transformer does not amplify uni¬ 
formly over this range some frequencies will be magnified more 
than others. This is evidently undesirable and prohibits true 
reproduction of voice or music. This fault is present in some 
transformers. 

447. One hears a great deal about “shielding the stages of 
an audio frequency amplifier to prevent interaction between the 
circuits and consequent howling. ,, For this reason, some trans¬ 
formers are carefully enclosed in an impenetrable housing of 
steel. The author has seen some German transformers which one 
would imagine were designed to withstand gunfire. In practice 
we have never appreciated the necessity for all these precau¬ 
tions unless more than three stages are used or unless there is 
















AMPLIFIERS 


87 


something radically wrong with the design of the transformer. 
If the transformer is properly designed, shielding is not needed 
in a two or three-stage amplifier. If the amplifier howls, it is 
usually caused by improper connection of the transformer ter¬ 
minals or by the capacity effect of unnecessarily long connect¬ 
ing wires. 

448. Fig. 84 is a photograph of a simple, well designed and 
constructed transformer with a 1 to 4 ratio, which we are now 
using with great success. Tests show that the amplification with 
this unshielded low-ratio transformer is greater than with most 



of the shell-proof high-ratio types. The amplification is even 
over the required range of frequencies, thus eliminating distor¬ 
tion of tones. 

449. This transformer is constructed in such a way that the 
leads to the terminals of the tube sockets can be made extremely 
short, which is a distinct advantage; long leads, with consequent 
capacity effects, are a frequent source of howling. Fig. 85 is a 
bottom view of the transformer suspended underneath a tube 



Fig. 85 


socket and clearly shows the very short connecting leads. In 
one of the receivers we shall describe later three of these trans¬ 
formers are used in a three-stage audio frequency amplifier and 
the spacing from center to center of each transformer is only 
about three inches. No shielding whatsoever is used and yet 
howling or distortion are never experienced. 

450. Transformer-coupled Radio Frequency Amplifier: The 
radio frequency amplifier using transformers to repeat the sig¬ 
nals from tube to tube is similar to the audio frequency amplifier 
of the same type except that the transformers are designed ac- 





THEORY OF RADIO RECEPTION 


cording to the frequency of the currents to be amplified. How¬ 
ever, while the audio frequency amplifier operates over a wide 
range of frequencies without being affected to any great ex¬ 
tent by the distributed capacity of the transformer coils the 
high frequency amplifier may be restricted to a very limited 
range of frequencies by the effect of distributed capacity. 

451. A step-up ratio transformer is out of the question for 
the capacity of the secondary coil would reduce the voltage 
across the grid circuit and diminish the amplification to almost 
zero. But even if the transformer is wound with a 1 to 1 ratio, 
the effect of the distributed capacity, as we already learned in 
connection with the inductance-coupled amplifier, would be to 
give each coil a resonant frequency within the range of fre¬ 
quencies to be covered by the amplifier. This would render the 
amplifier useless for practical purposes. To be of any use at 
all, it must magnify signals over a definite even though limited 
range of frequencies. 

452. Tuned Radio Frequency Amplifier: One way of over¬ 
coming this difficulty is illustrated in Fig. 86 which shows a 



circuit is provided with a variable tuning element so that the 
amplifier can be adjusted to magnify any frequency within a 
limited range by tuning each circuit to resonance with the in¬ 
coming signal. The variable condenser C3 is used to tune the 
external plate circuit of the amplifying tube and the condenser 
C4 to tune the input circuit of the detector tube. 

453. It is at once evident that while this arrangement is 
very efficient and might be used with only one stage of radio 
frequency amplification, the tuning adjustments would be too 
numerous with more than one stage. For instance, if a three- 
stage radio frequency amplifier of this type were constructed, 
there would be no less than seven or eight controls to be varied, 
each one tuning sharply and requiring careful adjustment. This 
would render the picking up of signals so difficult that the radio 
frequency amplifier would be impractical to operate. 

454. Modified Tuned R.F. Amplifier: The tuned amplifier, 
however, can be used in a modified form. With seven or eight 
controls tuning is impossible but if the controls can be reduced 
to not more than three variable elements without losing too much 
of the amplification, the system becomes practical. 
































AMPLIFIERS 


89 


455. Fig. 87 shows the wiring diagram of a complete re¬ 
ceiver with two stages of tuned radio frequency amplification and 
detector in which only three variable condensers are required 
for tuning. The antenna circuit and the external plate circuits 
of both amplifying tubes are untuned. The secondary circuits 
are tuned to resonance with the incoming signal by the three 
variable condensers, Cl, C2 and C3. 

456. In this system, since the primary circuits are not tuned, 
they are made aperiodic. That is to say, the coils LI, L3 and 
L5 are wound so that the resonant frequency of each of the pri¬ 
mary circuits is greater than the highest frequency to be ampli¬ 
fied. For instance, if the tuning of the secondary circuits covers 



Fig. 87 

the wave-length range of 200 to 600 meters, the natural wave¬ 
length (resonant frequency) of each of the primary circuits is 
made less than 200 meters. 

457. The object of this is to permit tight coupling between 
primary and secondary circuits and at the same time preserve 
selectivity, eliminating the necessity of a variable coupling con¬ 
trol. The coupling is made close to induce the highest possible 
e.m.f. in the secondary. But if the primary circuit has a resonant 
frequency within the range covered by the secondary tuning 
element the selectivity is destroyed and the amplification dimin¬ 
ished (see Chapter IV). The primary circuits are therefore 
made aperiodic. Additional amplification is obtained by the tight 
coupling because the reaction of the secondary upon the primary, 
when the secondary is tuned to resonance with an incoming sig¬ 
nal increases the resistance of the primary to this frequency and 
thereby magnifies the potential variations across the external 
plate circuit. In other words, the tuning of the secondary par¬ 
tially tunes the primary. 

458 The amplification obtainable by this system, of course, 
is not so great as when the primary circuits are also tuned to 
resonance but the design of a tuned radio frequency amplifying 
receiver is invariably somewhat of a compromise. The am¬ 
plification can only be increased by complicating the operation 
nf the svstem to such an extent that it becomes useless for 
practical work. The designer must take all the factors into con- 
si deration and find some combination which produces the great 
est amount of amplification without unduly complicating the 
operation. For instance, in Fig. 87 an antenna tuning condenser 
and a variable coupling between the. antenna and L2 C2 circuit 
are sacrificed because selective reception is assured by the tuning 
of the input circuits of the two succeeding tubes. In this way 
two controls are eliminated without any serious loss of efficiency. 




























90 


THEORY OF RADIO RECEPTION 


459. In short, the system of Fig. 87 is quite efficient. The 
amplification is good; the selectivity excellent; the operation 
simple. Provided the apparatus used in the circuit is well 
designed and constructed, a receiver based on this system of 
amplification will give very satisfactory results. 

In Part 2 practical information will be given concerning the 
construction of receivers employing this system of radio fre¬ 
quency amplification. 

460. The Lowell R.F. Transformer-coupled Amplifier: Late 
improvements in the design of radio frequency transformers have 
now greatly simplified the design and operation of radio fre¬ 
quency amplifiers. A two or three-stage high frequency ampli¬ 
fier operating with the stability of an audio frequency amplifier 
is now made possible by the use of a transformer designed 
and patented by P. D. Lowell, of the United States Bureau of 
Standards, Radio Division. Under the trade name of “DX” his 
transformer is already well known to the radio amateur. 

461. The Lowell transformer obviates the necessity of tun¬ 
ing the intermediate stages of a radio frequency amplifier. The 
capacity of the transformer is reduced to such a very low mini¬ 
mum by the patented design that the effect of this capacity upon 
the impedance of the primary and secondary coils is almost neg¬ 
ligible. The wave-length range covered by the transformer is 
determined by the inductance of the primary and secondary 
coils, the mutual inductance between them, and the capacity 
of the tubes in the amplifier. 

462. The Lowell or DX transformer undoubtedly provides 
the solution to the problem of amplifying the high frequencies 
of short waves in a simple, efficient manner. The simplicity 
and stability of the audio frequency amplifier are duplicated in 
the high frequency amplifier with DX transformers. Many imi¬ 
tations of the transformer have appeared on the market but 
none has equalled the original as devised by Mr. Lowell. 

463. The reason for the high efficiency of the DX trans¬ 
former, of course, lies in the almost total absence of capacity in 
the device. In his patent application, on which patent No. 
1,439,563 was issued Dec. 19, 1922, Mr. Lowell outlines the de¬ 
fects in earlier types of radio frequency transformers which were 
designed to eliminate the necessity of tuning the intermediate 
circuits of a high frequency amplifier. These earlier transform¬ 
ers were of two principal types; one of the shell type in which 
the primary and secondary windings were wound one beneath 
the other on the central core of a shell frame; and the other in 
which thin, flat coils were employed and the primary and sec¬ 
ondary arranged with their flat sides parallel. Both these types 
were inefficient, Mr. Lowell explained, because of the high dis¬ 
tributed capacity between turns, the capacity between primary 
and secondary and, in the first type, the capacity to the core of 
the transformer. All these capacity effects restrict the wave 
length range of the transformer and diminish the amplification. 

464. The construction of the DX transformer, designed by 
Mr. Lowell, and the manner in which this form of construction 
reduces to an absolute minimum all the capacity effects out- 


AMPLIFIERS 


91 


lined above, can best be comprehended by referring to the series 
of photographs of Figs. 88 and 95, which show the transformer 
in its different stages of construction. The particular type of 
DX transformer in these photographs covers the wave-length 
range of 250 to 585 meters. 

465. Coil Form (Fig. 88) : Formica tubing is used for the 
form on which the transformer is wound in order to ensure the 
best insulation and uniformity in construction. Slots are cut 
round the tubes to hold the coils. The slots are accurately made 
by carefully adjusted machines, so that there is no variation in 
thickness of spacing greater than one thousandth of an inch 



466. Completely Wound Form (Fig. 89) : The primary and 
secondary coils are wound with very fine copper wire in the slots 
cut in the coil form. The primary is wound in the six slots at 
one end of the form and the secondary in the six slots at the 
opposite end. The winding in each slot is completed before be¬ 
ginning the winding in the next adjacent slot. Each slot is 
wound with the same number of turns. Thus the primary and 
secondary coils each consist of a number of distinct and spaced 
windings connected in series with each other, each groove con¬ 
taining only a few turns of wire. In this way the inherent ca¬ 
pacity of the coils is made extremely low. Not only is the 
capacity between turns in each groove very small, but the total 
capacity across the entire coil is even smaller as the capacities 
of all six separate windings are joined together in series. 

The primary and secondary coils are separated from each 
other by a spacing of about $4". The spacing depends upon the 
wave-length range for which the transformer is designed as the 
spacing governs the mutual inductance between primary and 
secondary. In the short-wave transformer shown, the wide sepa¬ 
ration between primary and secondary renders the capacity be¬ 
tween them practically zero. 

Connecting wires are carefully soldered to the coil ter¬ 
minals and in soldering no paste or corrosive flux is used. This 


92 


THEORY OF RADIO RECEPTION 


eliminates the possibility of corrosion at the soldered terminals. 

467. Steel Core (Fig. 90) : The spacing between the primary 
and secondary coils of the transformer would be altogether too 
great and the coupling too loose if the transformer were of the 
air core type. But the core of the transformer is filled with 
strips of best sheet silicon steel. To keep the electrical losses 
negligible this steel is extremely thin and is carefully prepared 
before inserting in the transformer, thus eliminating all - pos¬ 
sibility of adjacent sheets becoming short circuited and reduc¬ 
ing the efficiency. Exactly the same quantity of steel is used in 
the core of each transformer. 



Fig. 90 


Fig. 91 


Fig. 92 


468. Assembled Transformer; Ready to Mount in Case (Fig. 

91) : The insertion of the steel core in the coil form makes the 
coupling between primary and secondary coils comparatively 
close without actually diminishing the distance between the two 
so that the capacity between the coils is still very low. The ca¬ 
pacity to the steel core is small as the coil form is quite thick 
and the windings are not close enough to the core to increase 
the capacity to any extent. Moreover, the insertion of the core 
greatly increases the inductance of both primary and secondary 
coils. Therefore the number of turns of wire on the transformer 
required to cover a given wave-length is much smaller than 
when an air core is used. The use of the steel core, therefore, 
still further reduces the distributed capacity of the windings 
by decreasing the necessary number of turns to obtain a given 
inductance. It is true that there are some losses inherent in a 
steel core high frequency transformer but these losses are much 
more than compensated by the elimination of capacity made pos¬ 
sible by the use of the steel core. 

469. Transformer Case (Fig. 92) : The outer case of the 
transformer is made of black formica. Good insulation and 
strength are just as essential here as in the winding form. Con- 


AMPLIFIERS 


93 


necting pins are rigidly fastened in the case, as shown in the 
photograph. 

470. Complete Transformer (Fig. 93) : The completely 
wound transformer is inserted in the case, the connecting wires 
from the coils soldered to the four pins and the case is com¬ 
pletely filled with a special highly insulating wax so that the 
transformer cannot be affected by moisture. The overall di¬ 
mensions of the transformer are: Height, 4"; Width, \y%” * 
Depth, 1 y 2 ". 


y 


Fig. 93 Fig. 94 

471. Plug-in Mounting (Fig. 94) : It is, of course, impos¬ 
sible to make a single radio frequency amplifying transformer 
that will operate at all the wave-lengths used in radio communi¬ 
cation. As we have seen, radio transformers are limited by their 
principle of operation to a comparatively narrow band of wave¬ 
lengths. Mr. Daniel of the Radio Instrument Co. realized the 
limitations of a radio frequency amplifier in this respect and pat¬ 
ented a plug-in mounting for the transformer. The four pins 
mounted on the back of the transformer casing plug into and 
make connection with four sockets on the transformer mount¬ 
ing. This feature makes it possible to remove transformers of 
one range and insert others having a different range if it is de¬ 
sired to hear signals on widely different wave-lengths. In this 
way a single amplifier can be used for reception at any wave¬ 
length merely by inserting the correct type of transformer. Only 
three types are required to cover the range from 200 to 3,000 
meters. 

472. Voltage Amplification of DX Transformer-coupled 
Amplifier: The heavy line curve of Fig. 95 shows the voltage 
amplification over a wide wave-length range actually obtained 
with a two-stage DX transformer-coupled amplifier using U.V. 
201 A tubes. The two dotted-line curves show the results ob¬ 
tained with two other standard makes of radio frequency trans¬ 
formers under the same conditions. 






94 


THEORY OF RADIO RECEPTION 


A comparison of these three curves establishes the superi¬ 
ority of the DX transformer. Not only is the voltage amplifica¬ 
tion much greater at all wave-lengths within the range tested, 
but the range of the DX transformer is much wider than either 
of the other two. 

The type of DX transformer with which the curve of Fig. 
95 was obtained is especially designed for use with U.V. 201 A 
tubes for the reception of radio broadcasting on wave-lengths 
from 250 to 585 meters. This one type of transformer there¬ 
fore covers efficiently all the wave-lengths' now used in radio 
broadcasting. 

The construction of amplifiers and complete receivers em¬ 
ploying DX transformers is fully expl ained in Part 2. 



CONTROLLING SELF-OSCILLATION IN A RADIO 
FREQUENCY AMPLIFIER. 

473. In multi-stage high frequency amplifiers of all types, 
whether they be resistance, inductance or transformer-coupled, 
there is often a tendency toward self-oscillation. For the recep¬ 
tion of damped waves and radio-telephony it is necessary, of 
course, to control these continuous oscillations—to prevent them 
from being generated. 

474. To understand the methods of accomplishing this it 
is first essential that we comprehend how the continuous oscil¬ 
lations are generated. We have already explained how regenera¬ 
tion and self-generated oscillations are produced by the reaction 
of the plate circuit of a tube upon its grid circuit through in¬ 
ductive or capacitive coupling. However, in a multi-stage am¬ 
plifier, while regeneration or self-oscillation may be produced 
by the feed-back of energy from the plate circuit of any tube in 
the amplifier back to the grid circuit of the same tube, similar 
results may be produced by the feeding back of energy from 
the plate or grid circuit of one tube to the plate or grid circuit 
of a preceding tube. 

475. It does not follow, however, that if any two circuits 
of an r.f. amplifier are coupled together, regeneration will be 
















































































































































AMPLIFIERS 


95 


produced. The coupling may have the directly opposite effect. 
In other words, energy may be fed back from one circuit to 
another in either a positive or negative sense. This can best be 
understood by considering the simple case of a regenerative re¬ 
ceiver using a tickler coil to produce regeneration or cause con¬ 
tinuous oscillations to be generated as in Fig. 67. The energy 
is here fed back by inductive coupling between the plate and 
grid circuits. The tickler coil and the coil in the grid circuit 
form a transformer. 

To produce regeneration and strengthen a signal oscillation, 
the e.m.f. induced in the grid circuit by the varying current in 
the tickler coil must act in the same direction as the signal oscil¬ 
lation. In other words, the oscillating e.m.f. induced in the grid 
circuit by the tickler coil must be in phase with the signal oscilla¬ 
tion. If the current does not pass through the tickler coil in 
the proper direction—that is to say, if the connections to the 
coil are reversed—the oscillating e.m.f. induced in the grid cir¬ 
cuit will be directly out of phase with the signal oscillation. 
Instead of strengthening the signal it will tend to damp it out. 
Instead of decreasing the resistance of the grid circuit it will 
increase the resistance. 

476. Now, in a multi-stage radio frequency amplifier energy 
may be fed back from one circuit to another through various 
forms of coupling. Some of these couplings are inherent and 
cannot be avoided. For instance, there is the capacity coupling 
between the plate and grid of each tube, or between some of 
the wiring; the inductive coupling due to magnetic linkages 
between transformers or coils in the amplifier; resistance coup¬ 
ling due to the internal resistance of the plate batteries; and 
possibly direct magnetic coupling caused by long wires connect¬ 
ing to the plate batteries. These sources of coupling are either 
unavoidable or can only be partially avoided. 

477. The energy which is fed back from one circuit to an¬ 
other in the amplifier through these various couplings may be 
either in a positive or negative sense. However, the positive 
feed-back is usually great enough to generate continuous oscilla¬ 
tions in the circuits. 

478. There are two methods employed to prevent the self- 
generation of continuous oscillations in a radio frequency am¬ 
plifier so that damped waves and radio telephone signals may be 
received without distortion. 

479. Controlling Self-Oscillations by Negative Feed-back: 
The first method, devised by the French Army during the war, 
is based on the principle of coupling two circuits of an ampli¬ 
fier together to produce a negative feed-back of energy which 
tends to neutralize the positive feed-back inherent in the ampli¬ 
fier. The two circuits are generally capacitively coupled through 

a small condenser. .... 

480. Fig. 96 shows one application of this principle in a 
resistance-coupled amplifier. In this application, both positive 
and negative feed-back are controlled. Resistance-coupled am¬ 
plifiers usually have less tendency towards self-oscillation than 
inductance or transformer-coupled amplifiers and, hence, in this 


96 


THEORY OF RADIO RECEPTION 


case, to produce continuous oscillations for the reception of un¬ 
damped waves, a positive feed-back control is also incorpor¬ 
ated. As the diagram shows, the grid of the first tube is con¬ 
nected to the movable plate of a special three-plate condenser. 
The two stationary plates of the “compensating’ condenser are 
connected respectively to the plates of the second and third am- 



481. Now, in 
each successive stage 
of a resistance- 
coupled amplifier the 
voltage is reversed in 
phase. That is to 
say, when the grid 
of the first tube is 
made positive by a 
signal impulse, the plate potential of the same tube drops. Con¬ 
sequently the grid of the second tube becomes negative and the 
plate positive; the potential of the third tube is then the same 
as the first; the fourth is the same as the second, and so on. 


482. In the system of Fig. 96, therefore, one fixed plate 
of the compensating condenser is wired to feed back positively 
to the grid of the first tube and the other fixed plate is wired 
to feed back negatively. The respective values of these two 
opposing feed-backs can be altered by turning the movable plate 
of the condenser. There is one position of the movable plate 
at which the negative feed-back neutralizes the positive feed¬ 
back. At any other position one feed-back is stronger than 
the other. In this way regeneration can be controlled and con¬ 
tinuous oscillations produced if desired. On the other hand, 
the negative feed-back can be in¬ 
creased to prevent self-oscillation. 

483. The same principle can be 
used in this or other ways to neu¬ 
tralize the positive feed-back of in¬ 
herent coupling in any type of radio 
frequency amplifier. Neutralizing 
feed-back can be produced by coupl¬ 
ing one plate to another or one grid 
circuit to another. 

484. For instance, in the trans¬ 
former-coupled r.f. amplifying cir¬ 
cuit of Fig. 97 (a) the plate of the 
second tube is coupled to the plate 
of the first tube by the small variable 
condenser Cl. With the radio frequency transformer Tr con¬ 
nected in the manner shown, the positive feed-back from the plate 
of the second tube will produce regeneration or self-oscillation 
if Cl is varied. But if the leads to the secondary of the trans¬ 
former Tr are reversed as shown in Fig. 97(b) the feed-back 
from the second tube is negative and will neutralize any inherent 
positive feed-back if Cl is properly adjusted. 













































AMPLIFIERS 


97 


Fig. 98 shows a negative feed-back from the grid of one 
tube back to the grid of the preceding tube, based on the same 
principle as Fig. 97(b). Some other 
feed-back arrangements of this kind 
can be found in British patent No. , 

127,014, issued November 7th, 1916. 

485. The Neutrodyne System: 

In the opinion of the author, the —-——- 

functioning of the so-called “Neutro- 98 

dyne” system in preventing the generation of continuous os¬ 
cillations in the circuits of a radio frequency amplifier, is ex¬ 
plained by the above paragraphs, particularly Paragraphs 483 
and 484. 



j 

f ( 


1 ( 

1 

w 




r 1 





400 Ohm Pot 


J Fig. 99 


Jr 


486. Potentiometer Method of Controlling Self-oscillation: 

The second method of controlling self-oscillation in a high fre¬ 
quency amplifier was developed by the British during the war 
and is shown in Fig. 99. Instead of connecting the grids of 

daries of the radio 
frequency transform¬ 
ers to a point of neg¬ 
ative potential on the 
-•filament circuit, they 
are connected to the 
sliding contact of a 
saqn; SuiXjqduiu aqi 
-uooos aq; qSnojqi 
potentiometer in parallel with the filament battery. By moving 
the potentiometer contact the normal potential of the grids with 
respect to the negative end of the filament can be varied from 
a negative value to a positive value. The principle of this 
arrangement was explained in Paragraph 325. The filament rheo¬ 
stats of the radio frequency amplifying tubes are connected be¬ 
tween the filament and the negative end of the filament battery 
so that by moving the potentiometer contact the grids may be 
made either negative or positive with respect to the negative 
end of the tube filaments. 


487. If continuous oscillations are generated when the po¬ 
tentiometer contact is at the negative end of the resistance the 
oscillations can be damped out by moving the contact towards 
the positive end. As the grids become more positive with re¬ 
spect to the filament the impedance of the grid-filament circuits 
is lowered. If the impedance is lowered sufficiently the grid- 
filament circuit becomes conductive. In either case the am¬ 
plification is diminished. More energy is consumed in the grid 
circuit; in other words, its resistance is increased. By properly 
balancing the resistance of the grid circuit against the feed-back 
self-oscillation can easily be controlled. 

488. There is some difference of opinion as to the respec¬ 
tive merits of the two methods of controlling regeneration and 
self-oscillation in a radio frequency amplifier. In either case, 



























98 


THEORY OF RADIO RECEPTION 


the self-oscillation is controlled by increasing the resistance 
of the circuits; in other words, by diminishing the amplification. 

489. The ideal radio frequency amplifier, of course, would 
be one in which all coupling between circuits (other than the 
coupling which repeats the signal oscillation from tube to tube) 
is entirely eliminated. Then there would be no feed-back in 
either a positive or negative sense. If this were possible any 
number of stages of amplification could be used and the signal 
oscillation enormously magnified. Regeneration could be added 
to still further magnify the signal. 

490. In a multi-stage amplifier this, at present, is impossible 
of accomplishment but the designer of a radio frequency ampli¬ 
fier should bend every effort to reach as close to this ideal as 
possible. All sources of coupling in the amplifier should be re¬ 
duced to a minimum. All inductances should be mutually at 
right angles to each other and separated. The apparatus should 
be arranged so that it may be connected together with very short 
wires—particularly the plate and grid leads. In this way the 
efficiency of the amplifier can be greatly improved as there is less 
tendency towards self-oscillation and the amplification need not 
be reduced to the same extent to stop self-oscillation. 

491. The amplifiers and complete receivers which are de¬ 
scribed in detail in Part 2 of this book are properly designed 
in this way to obtain maximum possible efficiency and high am¬ 
plification. 

492. These sets are divided into two classes: DX trans¬ 
former-coupled, and tuned radio frequency amplifying receivers. 
In the former class potentiometers are used to control self-oscil¬ 
lation. With more than one stage of DX transformer-coupled 
amplification this is found to be the most practical way to stabil¬ 
ize the operation of the amplifiers. 

493. The tuned radio frequency amplifiers use the system 
of coupling described in Paragraphs 454-459 of this chapter, 
and the receivers are designed in such a way that no potentio¬ 
meter or compensator is required. Continuous oscillations are 
not set up in the circuits; mainly because the most important 
source of feed-back coupling—the feed-back through the internal 
capacity of the tubes—is comparatively small as compared with 
the DX transformer-coupled amplifier. The oscillations set up 
across the resonant plate circuits of the DX transformer-coupled 
amplifier have a much higher amplitude than the oscillations set 
up across the aperiodic plate circuits of the tuned r.f. amplifiers. 
Therefore, with the latter type of amplifier the feed-back through 
the tube is much smaller. 

All other sources of feed-back coupling are eliminated by 
careful design and high amplification is secured without self¬ 
generated oscillations being set up. 

THE “REFLEX” SYSTEM. 

494. The so-called “Reflex” circuit can be usefully employed 
under certain conditions. This system was used by the French 


AMPLIFIERS 


99 


about seven or eight years ago, but has only recently been 
brought into prominence in this country. 

495. The principle of the system is shown in the diagram 
of Fig. 100. The signal oscillation is applied between the grid 
and filament of the amplifying tube A. The radio frequency 
current variations in the plate circuit of this tube set up oscilla¬ 
tions across the primary of the radio frequency transformer 
R.F.T. An oscillating e.m.f. is induced in the secondary of this 
transformer and the amplified oscillations applied between the 
grid and filament of the rectifying tube B. The audio frequency 
variations in the plate circuit of the rectifying tube induce an 
alternating e.m.f. across the secondary of the audio frequency 
transformer A.F.T. These amplified alternations are then ap¬ 
plied between the grid and filament of the amplifying tube A 
and the audio frequency variations of the plate current of this 
tube are detected by the telephones. Tube A, then, amplifies 
radio and audio frequency variations simultaneously. 



Fig. 100 


496. Provided the circuit is properly arranged there is no 
distortion of tone. Under certain conditions the amplification of 
both frequencies is quite good enough to warrant the use of 
the system. With only one amplifying tube the circuit is very 
satisfactory. The amplification at both frequencies is very g°°d. 
With more than one amplifying tube, each being used “twice,” 
considerable care has to be taken to avoid losses and distortion. 

497. The great advantage of the “reflex” system is the low 
cost of the receiver in which it is used. However, if a re¬ 
ceiver of the very maximum sensitiveness is desired, at a some¬ 
what greater cost, it is undoubtedly better to use separate tubes 
for radio and audio frequency amplification. 

498. The “reflex” system cannot be used to advantage in an 
r.f. transformer-coupled amplifier in which a potentiometer is 
necessary to control self-oscillation. The audio frequency am- 
plification is very poor when the grids of the amplifying tubes 
are not negative at all times with respect to the filament. 

499. “Reflex” is used in the tuned radio frequency amplify¬ 

ing receivers described in Part 2 and this feature greatly in¬ 
creases their efficiency. No potentiometer is used in these re¬ 
ceivers and the grids of the amplifying tubes are negative at 
all times. The radio and audio frequency amplification are 
both excellent. In fact, we will later describe a one-tube receiver 
which very successfully operates a loud speaker ( __ 













LESSON 8. 


AUDIBILITY AND SELECTIVITY OF RECEIVING 
SYSTEMS. 


500. In comparing the qualities of radio receiving systems 
or in determining the efficiency of a particular receiver, there 
are two qualities upon which judgment is mainly based, viz: 
“audibility” and “selectivity.” We have used these expressions 
already but in order that there may be no misunderstanding we 
shall define them as follows: 

501. Definition of Audibility: This has been defined as 
the ratio of the audio frequency current variations actually flow¬ 
ing through the telephones of a receiver to the current which 
must flow to make the signals just audible. For instance, if 
a certain signal picked up by a receiver sounds loud in the 
telephones, the current in the 'phones may be 100 times greater 
than the current which is necessary to render the signal just 
audible; in other words, the “audibility” is 100. 

502. The audibility of a receiver is proportional to the square 
of the amplitude of the e.m.f. signal oscillations impressed on 
the rectifying system. This will be understood by considering 
the rectifying system as explained in Paragraphs 302-309. 

503. When a signal oscillation is impressed on this rectify¬ 
ing system, increases of current are produced by positive im¬ 
pulses and decreases of current by negative impulses. If the 
increases are greater than the decreases, rectification takes place; 
the average current through the telephones or output circuit of 
the rectifying system is greater than before. But the extent of 
the current increase depends upon how much greater are the in¬ 
creases than the decreases. In other words, the effective value 
of the rectified current depends upon the difference between the 
increases of current produced by positive impulses of the signal 
oscillation and the decreases of current produced by negative 
impulses. If the amplitude of the signal oscillation itself is in¬ 
creased, this difference increases but they do not increase in the 
same proportion. For instance, if the amplitude of the signal 
oscillation is increased five times the effective value of the rec¬ 
tified current is increased about twenty-five times. 

504. If a signal is so weak that there is no difference be¬ 
tween the increases of current produced by positive impulses 


RECEIVING SYSTEMS COMPARED 


101 


and the decreases of current produced by negative impulses, 
the signal is not rectified and is inaudible. The audibility in¬ 
creases, however, as the square of the amplitude of the signal 
e.m.f. increases, until the saturation point is reached. Above 
a certain amplitude of signal e.m.f. the rectified current is not 
increased by an increase of impressed e.m.f. 

505. Without either radio or audio frequency amplification, 
then, the audibility of a simple detecting system is greater for 
strong signals than weak signals. Distant stations and weak 
transmitters may be entirely inaudible. 

506. The addition of audio frequency amplification increases 
the audibility of signals which are strong enough to operate the 
rectifier but has no effect upon the signals which are too weak 
to be rectified. 


507. The addition of radio frequency amplification, on the 
other hand, increases the audibility of weak signals more than 
strong signals. If a signal is already so strong that it can pro¬ 
duce the maximum possible output from the detecting system, 
radio frequency amplification does not increase the audibility 
at all. But if the signal is very weak, the radio frequency am¬ 
plifier magnifies the signal oscillations and greatly increases the 
audibility. For instance, a two-stage DX transformer-coupled 
amplifier magnifies the voltage of a 400-meter signal about 13 
times. This will increase the audibility of a weak 400-meter sig¬ 
nal about 169 times (13 2 ). * 

508. Definition of Selectivity: This can be defined as the 
ratio of the frequency to which a receiver is tuned to the differ¬ 
ence between this frequency and the frequency of another signal 
of the same strength which is just audible. For instance, if a 
receiver is tuned to resonance with a signal of 1,000,000 cycles 
(300 meters) and another signal of the same intensity on. 750,000 
cycles (400 meters) is just audible, the selectivity is given by 
the ratio: 

1,000,000 

___= 4. 


1,000,000 — 750,000 

The higher this ratio the better is the selectivity. 

509 In the above case the selectivity is poor because, while 
the receiver is tuned to 1,000,000 cycles (300 meters), all signals of 
the same intensity from 750,000 cycles (400 meters) to 1,250- 
000 cycles (240 meters) are audible. The audibility of the 300- 
meter signal, of course, is greatest, since the receiver is tuned 
to the frequency of this signal. But another signal of greater 
intensity than the signal to which the receiver is tuned may 
be as loud or louder than the desired signal even though the 
frequency of the interfering signal is greater or less than the 
510. In order that this may be quite clear, let ustake an¬ 
other example: Suppose a receiver is tuned to 1,000,000 cycles 
(300 meters) and another signal of the same intensity on 900,000 
desired signal. 



102 


THEORY OF RADIO RECEPTION 


cycles (333 meters) is just audible, the selectivity ratio can be 
written: 1,000,000 

-= 10 . 

1,000,000 — 900,000 

This receiver has a little better selectivity than the first (the 
ratio is higher). All signals of the same intensity from 900,000 
cycles (333 meters) to 1,100,000 cycles (272 meters) are audible, 
but the 300-meter signal, of course, is loudest since the receiver is 
tuned to resonance with this frequency. Again, however, a 
stronger signal above or below the resonant frequency may 
be as loud or louder than the desired signal. 

511. Audibility vs. Selectivity: While there are several 
practical factors which must be taken into consideration when 
comparing radio receiving systems—such as ease of operation, 
cost, etc.—it is apparent that the comparison must be largely 
based on the audibility and selectivity possessed by the respec¬ 
tive systems. If a receiver has good audibility but poor selectiv¬ 
ity, it is of little use. There is no advantage in being able to 
hear hundreds of stations within a range of two thousand miles 
or so if one hears them all at the same time. On the other 
hand, if a receiver has extremely high selectivity but very poor 
audibility it may be equally useless. 

512. Sometimes these two qualities are inversely propor¬ 
tional to each other. If the audibility of a receiver is increased 
the selectivity may be decreased in the same proportion and 
vice versa. If this condition exists the designer or the operator 
of the receiver, as the case may be, should seek to obtain the 
highest possible selectivity compatible with reasonable audi¬ 
bility. 

513. If the audibility of a receiver is increased by adding 
regeneration or radio frequency amplification the selectivity must 
be proportionately increased. When the audibility is increased 
the range of the system is widened. If the selectivity is not in¬ 
creased the added audibility is more of a drawback than an 
advantage. It merely means that more stations are heard than 
before; in other words, that interference is increased. 

514. Suppose, for instance, that an amateur adds a radio fre¬ 
quency amplifier to his receiving system and the type of ampli¬ 
fier is such that the audibility alone is increased. With his old 
set he was using a moulded vario-coupler or some such arrange¬ 
ment to vary the coupling between his antenna and secondary 
circuits. This coupler probably afforded sufficient control of 
audibility and selectivity with his old set because the audibility 
was comparatively low. He was only able to hear stations within 
a radius of, say,. 250 miles. Retaining the same tuning arrange¬ 
ment, the addition of the radio frequency amplifier widens his 
range of reception to include all stations within, say, 1,500 miles. 
Hundreds of stations he has never even heard on his old set now 
come in almost as strongly as the local stations. But, as the 
selectivity of his receiver has not been increased in any way, 
he hears them all at the same time —he cannot tune one from 
another. The solution is obvious. The selectivity of the tuner 
which precedes the amplifier must be increased. 



103 


RECEIVING SYSTEMS COMPARED 

RECEIVING SYSTEMS COMPARED. 

515. Single Circuit Non-Regenerative Receiver: We saw, in 
Lesson 4 that the resonance curve of an oscillatory circuit 
demonstrates the selectivity of the circuit as well as the value 
of the current which flows in the circuit when an oscillating 
e.m.f. is induced in it. The sharper this curve the greater is 
the selectivity and the higher is the maximum current value 
at the resonant frequency. We also learned that a resonance 
curve is flattened out by the effect of resistance in an oscillatory 
circuit. . The simple non-regenerative system has only one oscil¬ 
latory circuit, namely the antenna circuit. The resonance curve 
of this circuit determines the selectivity and the audibility of the 
system. Owing to the comparatively high resistance of the an¬ 
tenna, this system has poor selectivity and poor audibility. 

516. Inductively-Coupled Non-Regenerative Receiver: In 
this system, as shown in Fig. 66, there are two oscillatory cir¬ 
cuits for tuning purposes—the antenna circuit and the secondary 
closed circuit. The selectivity and audibility of the system can 
be adjusted by varying the coupling between the two oscillatory 
circuits, each circuit being tuned to resonance. For a given re¬ 
sistance of circuits there is one low value of coupling at which 
the audibility is maximum (see Paragraph 276). If the coupling 
is made closer than this value the audibility and selectivity are 
both decreased; with weaker coupling the selectivity increases 
as the audibility decreases. 

517. Inductively-Coupled Regenerative Receiver: In this 
type of receiver, as depicted in Fig. 70, there are three oscilla¬ 
tory circuits; the antenna, the secondary, and the tuned plate 
circuit. The regenerative action reduces the effective resistance 
of the secondary circuit. This, of course, makes the audibility 
and selectivity much higher than with the non-regenerative re¬ 
ceiver of the same time. 

518. The audibility and selectivity can be controlled by 
varying the coupling between the antenna and secondary cir¬ 
cuits. The non-regenerative receiver may require a coupling 
of six or seven per cent to secure maximum audibility but the 
degree of coupling which gives maximum audibility with a well- 
designed regenerative receiver may be as low as three per cent 
and should never be more than five per cent. This automatically 
makes the inductively-coupled regenerative receiver very selec¬ 
tive. Signals within two or three meters of the desired signal 
are inaudible. The coupling between the antenna and the sec¬ 
ondary circuit is so low that interfering signals which may set 
up forced oscillations in the antenna circuit are not transferred to 
the secondary. Even if they are strong enough to be trans¬ 
ferred, the tuning of the low resistance secondary circuit is so 
extremely sharp that the interfering signal is damped out. 

519. And yet this very loose coupling (from three to five 
per cent, as the case may be) gives maximum audibility to sig¬ 
nals of the desired frequency. If the coupling is made closer, 
the signal strength and selectivity are both decreased. Obvi¬ 
ously, there is no advantage in making the maximum coupling 


104 


THEORY OF RADIO RECEPTION 


of a radio-coupler greater than five per cent if it is intended 
for a regenerative receiver. The only useful variation is between 
zero and five per cent; the coupling which gives maximum audi¬ 
bility can be found within these limits. 

520. Semi-tuned Transformer-coupled R.F. Amplifying Re¬ 
ceiver : If radio frequency amplification is used in a receiver, the 
signal oscillations are impressed on the amplifier and repeated 
from tube to tube by means of tuned or semi-tuned oscillatory 
circuits (except in the case of the resistance-coupled amplifier). 
The resonance curves of these circuits manifestly affect the audi¬ 
bility and selectivity of the system, while the amplifying action 
of the tubes greatly increases the audibility. 

521. The semi-tuned transformer-coupled type of amplifier, 
in which DX or similar transformers are used, is inherently non- 
selective; in fact, it is designed to be non-selective. The whole 
object of the transformers is to eliminate the necessity of closely 
tuning each intermediate circuit of the amplifier and the trans¬ 
formers are purposely made to cover as broad a range of fre¬ 
quencies as possible. In Fig. 95 we showed the voltage amplifi¬ 
cation of a two-stage DX transformer-coupled amplifier against 
the wave-length range. An amplifier with these transformers 
magnifies all signals from 250 to 585 meters; some waves are 
amplified more than others, but they are all covered. Other 
transformers of similar design have the same effect, although 
they do not accomplish it so efficiently. 

522. This type of amplifier, then, enormously increases the 
audibility of a receiver but in no way increases the selectivity. 
However, we are not implying that these amplifiers are ineffi¬ 
cient. On the contrary, considerable advantage can be taken 
of the enormous increase in audibility they afford if the selec- 



Fig. 101 


tivity of the tuner which precedes the amplifier is increased. 
By properly designing this tuner, the selectivity and audibility 
can be controlled with unusual efficiency in a very simple 
manner. 

523. Fig. 101 shows a typical receiving circuit with a tuner, 
a two-stage DX transformer-coupled r.f. amplifier, a detector and 
two-stage a.f. amplifier. This arrangement is very popular to¬ 
day. It is also very efficient, if the tuner is made super-selec¬ 
tive. But if the tuner is non-selective, the tuning is so broad 
that the system is almost useless. 







































RECEIVING SYSTEMS COMPARED 


105 


524. What type of tuner, then, should be used with a semi- 
tuned transformer-coupled amplifier? To be of practical use, it 
must be simple to operate. There would be no point in care¬ 
fully eliminating the tuning of a radio frequency amplifier to 
reduce the controls and then using five or six controls in an 
elaborately selective tuner. In fact, the active controls should 
number not more than three. 

525. The first arrangement that suggests itself is the in¬ 
ductively-coupled tuner with a tuned antenna circuit, tuned 
secondary circuit and a variable coupling between the circuits— 
three controls. But is this system selective enough? If an 
interfering signal is not damped out by the tuner, it will pass 
into the radio frequency amplifier and be magnified together 
with the desired signal. There are no sharply tuned circuits 
in the amplifier to diminish the interfering signal. The tuner 
must be ultra-selective and just pass signals of the desired 
frequency into the amplifier to be magnified. 

526. Judging from the results obtained with vario-couplers 
of standard make, this is more than can be expected from the 
simple tuning arrangement suggested above. But the fault lies 
in the design of the vario-couplers and not in the principles of 
the system. 

527. We have already discussed these principles. In com¬ 
mon with the simple regenerative receiver, when an inductively- 
coupled tuner is followed by a radio frequency amplifier, the 
value of coupling between antenna and secondary circuits which 
gives maximum audibility is very low—three to five per cent. 
With both types of receivers, if the coupling is increased above 
the low value which gives maximum audibility, the selectivity 
and audibility are both decreased. In either case, nothing is 
gained by increasing the coupling beyond this value. But. if 
the couplng is decreased below the point of maximum audibility 
the audibility decreases as the selectivity increases. If reference 
is made to the curve of Fig. 51, it will be seen that this decrease 
in audibility with proportionate increase in selectivity is very 
sharp between about five per cent and zero. 

528. Now, the amplification of a regenerative receiver is 
definitely limited and the signal strength is unnecessarily, di¬ 
minished if the coupling is reduced below the degree which gives 
maximum audibility. The selectivity, at this degree of coupling 
is quite good enough for the audibility which the regenerative 
receiver provides. The vario-coupler for the regenerative re¬ 
ceiver should merely be designed to make easy the location, of 
the degree of coupling between three and five per cent, which 
gives maximum audibility. 

529. But whereas the amplification by regeneration reaches 
a definite limit, the magnification produced by the radio fre¬ 
quency amplifier is greatly in excess of this limit. By sacrificing 
a little audibility the coupling can be reduced even below three 
per cent to gain extreme selectivity. Three per cent coupling is 
very selective and this may be the coupling which gives maxi¬ 
mum audibility, but below this the selectivity is incredibly high. 
There is no disadvantage in sacrificing some of the audibility to 


106 


THEORY OF RADIO RECEPTION 


increase the selectivity. The audibility is so enormously in¬ 
creased by the radio frequency amplifier that it is better to con¬ 
centrate all the amplification on a single wave-length rather 
than increase the audibility and spread the amplification over a 
broad band of wave-lengths. 

530. We can say, then, that to adjust the audibility and se¬ 
lectivity of either a regenerative receiver or a semi-tuned, trans¬ 
former-coupled radio frequency amplifying receiver with maxi¬ 
mum efficiency, the vario-coupler must provide a variation of 
coupling between zero and five per cent. Moreover, it must be 
possible to closely adjust the coupling between these limits as a 
slight variation of coupling between zero and five per cent has a 
large effect upon the audibility and selectivity. The same type 
of coupler serves for either system of reception as the require¬ 
ments for both are almost identical. 

531. And now, do the ordinary vario-couplers of standard 
make fulfil the requirements of this type of tuning? We have 
tested a score or more of couplers of different makes and can 
unhesitatingly say that these requirements are not filled. With 
the average coupler it is impossible to reduce the coupling to 
zero, or even close to zero; moreover, the maximum coupling is 
usually away beyond five per cent. One reason for this is quite 
apparent. The couplers are usually designed so that the rotor 
revolves inside the primary winding with only a minute spacing 
between the two. When the rotor is parallel with the primary 
coil, the coupling, both capacitive and inductive, is maximum, 
and this maximum is far above five per cent—too high for re¬ 
generative or radio frequency amplifying receivers. When the 
rotor shaft is revolved only 90 degrees so that the rotor is at 
right angles to the primary coil, the inductive coupling is al¬ 
most zero, but the capacitive coupling between the coils is quite 
high. 

This is the most useless type of coupler. So much care and 
attention are devoted to polishing up the beautiful moulded 
forms to make the finished product look more like something 
g;ood to eat than an electrical instrument that insufficient atten¬ 
tion is paid to the electrical characteristics. 

532. Other couplers are a little better designed and are 
suitable for non-regenerative receivers which require a closer 
coupling than five per cent. The rotor is not jammed up close 
to the primary but is fairly widely spaced from it. However, the 
maximum and minimum degrees of coupling are still too high 
for use with regenerative or r.f. amplifying receivers. Only a 
small portion of the rotor revolutions can be usefully employed. 
Why be limited to only a five or ten-degree revolution of the 
rotor shaft with the rest of the revolution useless and the mini¬ 
mum coupling too high? 

533. Using even the best types of standard vario-couplers, 
one is limited to a minute revolution of the coupling dial to 
adjust one of the most important tuning controls. The slightest 
movement of the coupling dial may change the coefficient of 
coupling near the minimum point sufficiently to cut down the 
audibility 100 per cent. This naturally makes tuning so difficult 


RECEIVING SYSTEMS COMPARED 


107 


that it is impossible to gain selectivity or control audibility. To 
take advantage of the control of selectivity and audibility of¬ 
fered by the vario-coupler system, it must be possible to closely 
adjust the coupling from zero to not more than five per cent. 

534. The Harkness Coupler: Realizing the need for a 
coupler which fills these requirements the writer designed one 
for the purpose. A photograph of it appears in Fig. 102. The 
maximum coupling afforded by this coupler is not more than 
five per cent, while the minimum is zero. Moreover, it requires 
a 180-degree revolution (complete half-turn) of the rotor shaft 
to cover this range of coupling. 



Fig. 102 

535. The construction of this coupler is plainly shown in 
the photograph. The primary is wound on the lower end of a 
long cylindrical form while the secondary is wound on a smaller 
coil which revolves inside the primary form but at the opposite 
end from the primary winding. The rotor shaft is set at an 
angle to permit a 180-degree variation of coupling. 

536. When the rotor is parallel with the primary coil the 
coupling is maximum but, from center to center, the spacing be¬ 
tween the two coils is almost four inches. This maximum 
coupling is only about five per cent. When the rotor shaft is 
revolved through an angle of 180 degrees the secondary coil 
itself turns from the parallel position until it is at right angles to 
the primary coil. Here the inductive coupling is zero and, owing 
to the wide spacing between the coils, the capacitive coupling is 
also zero. 

537. With this coupler, the adjustment of audibility and 
selectivity is a simple matter. Instead of being restricted to 
a minute variation of the coupling dial the whole 180-degree 
revolution can be utilized. When the coupler is used with a 
regenerative receiver the exact coupling which gives maximum 
audibility with high selectivity can easily be located. When it 
is used with a one, two or three-stage radio frequency am¬ 
plifying receiver the exact degree of coupling necessary to 
eliminate an interfering signal can similarly be found with ease. 
In actual practice the device has proved most successful. 





108 


THEORY OF RADIO RECEPTION 


538. The tuned plate regenerative receiver (as in Fig. 70), 
and the radio frequency amplifying receiver with broad reson¬ 
ance curve transformers are both widely used today. The effi¬ 
ciency of these receivers can be increased 100 per cent by the 
use of this specially designed coupler. There is no change in 
the wiring. The coupler is “hooked up” in the same way as any 
other vario-coupler in accordance with the wiring diagrams of 
standard regenerative and radio frequency amplifying receivers. 

539. This special coupler is used in the DX transformer- 
coupled r.f. amplifying receivers described in Part 2. The use 
of this coupler in these sets contributes in large measure to their 
unusual efficiency and simplicity of operation. 

540. Tuned Transformer-coupled R.F. Amplifying Receiv¬ 
ers: Receivers using this method of radio frequency amplifica¬ 
tion are inherently selective and it is because of this selectivity 
that it is possible to dispense entirely with a variable coupling 
between the antenna circuit and secondary circuit. The radio 
frequency amplifier itself ensures selectivity. Each stage of the 
amplifier tunes quite sharply. In the two-stage amplifier of Fig. 
87 there are three tuned circuits each sharply tuned to the in¬ 
coming signal frequency. When these three circuits are adjusted 
for maximum audibility—when each is tuned accurately to 
resonance—the selectivity is very good. 

541. For instance, if the amplifier is tuned to receive a 400- 
meter wave and another signal on 410 meters is impressed on the 
receiving antenna the 400-meter signal is enormously magnified, 
whereas the 410-meter signal is greatly diminished by the re¬ 
actance of each of the tuned circuits. Each stage of the am¬ 
plifier increases the amplitude of the 400-meter signal and re¬ 
duces the amplitude of the 410-meter signal. When the two 
signals are finally impressed on the rectifying system the ampli¬ 
tude of the 400-meter signal is very much greater than the 
amplitude of the 410-meter signal; in fact, the latter may be 
completely damped out. 

542. With the tuned transformer type of amplifier, then, 
the selectivity increases as the audibility increases. The more 
accurately the circuits are tuned to resonance, the sharper the 
resonance curve of each circuit; the more stages of amplification 
used—the greater become both the audibility and selectivity. 
Moreover, the selectivity is automatically increased by an in¬ 
crease of audibility; it is not necessary to sacrifice audibility 
to gain selectivity. 

LOOP ANTENNA RECEPTION. 

543. Principles of Loop Reception: The closed coil antenna, 
or “loop,” was first used as a radio compass to determine the 
direction of a transmitting station and it is still principally 
used for this purpose today. All'along the coasts radio com¬ 
pass stations with loop antennae greatly assist navigation by 
giving bearings to ships at sea. However, a loop can also be 
used with a radio frequency amplifier to receive radiophone 
broadcasts and amateur transmitters if it is inconvenient to erect 
an outdoor aerial. 


RECEIVING SYSTEMS COMPARED 


109 


544. The easiest way to understand the principles of loop 
reception is to consider the loop as the secondary inductance 
of an inductively-coupled receiving system of which the trans¬ 
mitting station antenna is the primary circuit. A loop is merely 
a large inductance coil—nothing else. Shunted by a variable 
condenser it forms a closed oscillatory circuit. This closed 
circuit is connected across the grid and filament of the first 
radio frequency amplifying tube in exactly the same manner as 
the secondary circuit of an inductively-coupled receiver; it is 
similarly tuned to resonance with a desired signal by means 
of the variable condenser. No primary antenna circuit is used 
at the receiving station. The transmitting antenna may be con¬ 
sidered the primary circuit. The inductive coupling between the 
receiving loop circuit and the transmitting antenna circuit can 
be varied from zero to maximum by revolving the loop just as 
the rotor of a vario-coupler can be revolved to vary the coupling 
between the primary and secondary circuits of a loose-coupled 
receiver. It is this latter feature of the loop antenna which 
makes it extremely directional. 

545. To receive any ^ ^ 

particular transmitting sta- / / j' / ^ -^ 

tion the loop must be , . / , / --- \ \ 


Fig. 103 the point A repre- x 

sents the transmitting antenna and the broken lines are the magnetic 
fields of the waves radiated by the transmitting station. The 
loop B is pointing in the direction of the transmitter A. The in¬ 
ductive coupling between A and B is maximum; therefore, the 
audibility of the receiver using B as antenna is maximum. 

The loop C is turned at right angles to the direction of wave 
travel from A. The inductive coupling between A and C is zero; 
therefore, the audibility of the receiver using C as antenna is 
zero. Signals from A are inaudible. 

547. Of course, the loop C may be pointing in the direction 
of a second transmitting station and the audibility of the signals 
from this second station may be maximum. In this case the 
signals from A do not interfere with reception. The directional 
characteristics of a loop may be utilized in this way to gam 
selectivity. 

548 Figs 104 A and 104 B show a collapsible loop antenna, 
designed by one of the Radio Guild engineers, which is eminently 
suitable for the reception of wave-lengths from 200 to 600 


pointed in the direction of 
the station. If the loop is 
turned through an angle of 
90 degrees so that it is at 
right angles to the direction 
of the transmitting station, 
the inductive coupling be¬ 
tween the two stations is 
zero and no signals are 
heard. 

546. For instance, in 



Fig. 1 OS 




110 


THEORY OF RADIO RECEPTION 


meters. This loop is a large inductance coil wound on a frame 
four feet square. An interesting feature of the design of this 

loop is the fact that the bakelite 
cross-pieces can be turned to change 
the loop from a solenoid to a spiral 
if desired. The loop can be turned 
in any direction. 

549. Efficiency of Loop Re¬ 
ception: The loop has been used 
by many amateurs to gain selec¬ 
tivity. These amateurs had added 
radio frequency amplification to 
their receivers and were disap¬ 
pointed by the broad tuning which 
their sets developed. The loop 
antenna was suggested by radio 
magazines and others as a solu¬ 
tion to this difficulty and was 
adopted by many. Immediately 
their receivers became selective 
again and the conclusion was 
reached that an outside aerial can¬ 
not be efficiently used with a radio 
frequency amplifier. 

This conclusion, of course, was erroneous; although, until 
the coupler described in Paragraph 534 appeared on the mar¬ 
ket, it must be admitted there was no simple and efficient method 
of gaining selectivity. 

550. The loop antenna undoubtedly makes possible very 
selective reception. It accomplishes this in two ways: 

1. By its directional characteristics ; 

2. By enormously decreasing the audibility of the re¬ 
ceiving system with which it is used. 

551. The second is by far the more important factor. For in- 



Fig. 1045 


stance, if an amateur adds a broad tuning radio frequency ampli¬ 
fier to his receiver the audibility is enormously increased without 
any increase in selectivity. With an outside aerial he hears 
stations within a range of 2,000 miles or so, but the selectivity 
of the receiver is so poor that interference renders it practically 
useless. To gain selectivity he uses a loop antenna. Immedi¬ 
ately the audibility is enormously decreased and selective recep¬ 
tion is made possible. But the receiving range of a two-stage 
radio frequency amplifier and detector with a loop antenna is 
little or no greater than a regenerative detector alone with an 



Fig. 104 A 



RECEIVING SYSTEMS COMPARED 


111 


outside aerial. He has gained nothing by adding the radio fre¬ 
quency amplifier to his receiver. 

552. However, if this amateur substitutes a Harkness Coup¬ 
ler for the coupler in his tuner selective reception with an out¬ 
side aerial is made possible and the advantages of the aerial 
and the radio frequency amplifier retained. The coupler per¬ 
mits accurate control of audibility and selectivity. If, to elim¬ 
inate an interfering signal, it is necessary to decrease the audi¬ 
bility of the system, the audibility can be very gradually de¬ 
creased by revolving the coupler dial. The slightest decrease 
of audibility, with its proportionate increase of selectivity, may 
be sufficient to eliminate the interfering signal. 

553. Of course, there are many occasions on which a loop 
must be used. But if the object of the radio frequency amplifier 
is to increase the range of a receiving system, the outside aerial 
must be retained and the Harkness Coupler used to gain selec¬ 
tivity. If a loop is used instead of an aerial the range is not 
increased (unless, of course, three or more stages of r.f. ampli¬ 
fication are employed). 

554. A loop antenna, then, should only be used when it 
is inconvenient to erect an outside antenna. To operate effi¬ 
ciently, of course, a radio frequency amplifier must be used with 
a loop or the receiving range is very limited. 

555. Incidentally, the range of a receiving system with loop 





To filament 


Fig. 105 


antenna and radio frequency amplifier is a fairly good indica¬ 
tion of the efficiency of the receiver. If,, with a small loop an¬ 
tenna, one can pick up signals from stations say, one thousand 
miles distant, the receiver is very sensitive. If an outside aerial, 
with the Harkness Coupler as tuner, is used in place of the loop 
antenna, the range will be more than doubled. 

556. Combining Loop and Aerial Reception: Fig. 105 shows 
a convenient way of combining loop reception and outside aerial 



































































112 


THEORY OF RADIO RECEPTION 


reception in one complete receiver. A telephone jack is con¬ 
nected in the receiver as shown in the diagram; the terminals of 
the loop are connected to a telephone plug. When the plug 
is inserted in the jack the loop takes the place of the rotor 
of the Harkness Coupler. The secondary tuning condenser is 
used in the usual manner to tune the loop circuit. When the 
plug is removed from the jack the rotor of the Harkness Coupler 
is again connected across the secondary condenser. In this way 
either the loop or the outside aerial can be used for reception. 
Fig. 106 shows the antenna circuit tuned by varying only the 
inductance of the coupler primary, but better results are ob¬ 
tained if the antenna is tuned with a variable condenser. 

557. Fig 106 shows the design of a very simple loop an¬ 
tenna which is especially adaptable for use with the system of 



Fig. 106. A telephone plug is attached to the lower end of the 
vertical stick. The whole loop can then be '‘plugged” into the 
jack in the receiver, the latter being secured to a shelf panel so 
that the loop can be supported in an upright position. The de¬ 
sign of this loop is plainly indicated in the drawing of Fig. 107 
and photographs of it appear in Part 2. The wire is wound in 
a spiral form on supports fastened to the bakelite strips. The 
ends of the wiring are brought down to the terminals of the plug. 


















PART 2 


Construction of 
Radio-Audio Frequency 
Amplifiers and 
Complete Receivers 








































































. 











. 

- 

. 





























































PART 2 


LESSON 9. 

RADIO-AUDIO FREQUENCY AMPLIFYING UNITS. 


With the knowledge gained in the first part of this book of 
the theory of operation of different types of radio receiving sys¬ 
tems and the comparison of their respective merits which was 
made in the eighth lesson, the reader should be able to reach 
his own conclusions regarding the system of reception which is 
best suited to his needs. 

In these remaining lessons we are giving complete details 
of the construction and operation of the better types of ampli¬ 
fiers and receivers, the principles of which were discussed in 
Part 1. These sets all employ radio frequency amplification. 
Some are exceedingly simple and inexpensive; others are more 
elaborate, more sensitive and consequently somewhat more ex¬ 
pensive. We all like to have “the best/’ but of necessity, the 
best costs more. Fortunately, even the best radio receiver can 
be made by the home constructor at a much lower cost than the 
price at which it can be purchased. By making his own re¬ 
ceiver, the amateur eliminates many “overheads” and profits 
which must be charged against the completely manufactured 
article. 

The photographs and drawings which appear on these pages 
will give the reader an intimate knowledge of how commercial 
receivers of the finest type are designed, built and wired. With 
the assistance of the instruction matter in the text, the duplica¬ 
tion of these receivers in both appearance and operation should 
prove simple even to one with but slight mechanical ability. 

We have occasionally read statements that radio frequency 
amplifying receivers are unstable, “liable to break into oscilla¬ 
tion without any particular reason,’ and so forth. If this be true, 
the particular receivers described must have been improperly 
designed, carelessly wired or composed of cheap and unsuit¬ 
able apparatus. The sets which we describe in this book have 
no such distressing peculiarities. They are in every way prac¬ 
tical and simple to operate. In fact, they are more stable and 
simpler to operate than most types of receivers. 



116 


CONSTRUCTION OF RECEIVER 


If good results are to be expected, the apparatus used in 
the construction of a radio receiver must be of the finest qual¬ 
ity and must be designed properly electrically as well as mechan¬ 
ically. A noisy or poorly designed rheostat or a tube socket 
with loose spring contacts will be a constant source of trouble. 
A poorly designed audio frequency transformer will cause howl¬ 
ing. The electrical losses in some types of variable condensers 
and other essential apparatus are quite high and may lower 
considerably the efficiency of the receiver. Moreover, if the re¬ 
ceiver itself is not designed properly it will not function at its 
best, even if perfect apparatus is used. As we suggested in Para¬ 
graph 490 of Part 1, a radio frequency amplifier must be designed 
and wired in such a way that all capacitive, inductive, or re¬ 
sistive coupling which would feed back energy from one cir¬ 
cuit to another must be reduced to the lowest possible minimum. 

All the faults and causes of instability of highly sensitive 
receivers have been brought to the attention of the author in 
the most effective possible manner. Hundreds of receivers have 
passed through our hands for testing and approval. This test¬ 
ing work has proved most useful in the correction of faulty de¬ 
sign or apparatus. For instance, howling was often present in 
the receivers with three stages of audio frequency amplification. 
The “howl” had to be removed from each set before it passed 
inspection. Almost every known make of transformer was tested 
in an attempt to make the receiver without the necessity of 
later changing it to remove howling. We are now using a trans¬ 
former which gives high amplification without a trace of howling 
or distortion. This source of trouble was permanently corrected 
by the use of this transformer. 

By following the instructions given in these pages and using 
the apparatus suggested, the amateur constructor will experi¬ 
ence none of the difficulties suggested above. These problems 
have all been effectually solved and the receivers herein set forth 
represent the solution. 

UNIT SYSTEM OF RADIO-AUDIO FREQUENCY 
AMPLIFIERS. 

The number of stages of radio or 
audio frequency amplification which can 
be usefully employed in a radio receiver 
is limited. If more than three stages of 
r.f. amplification are used, it is extremely 
difficult to control self-oscillation. Simi¬ 
larly, three stages of audio frequency am¬ 
plification produce extremely loud sig¬ 
nals—loud enough for ordinary purposes. 
However, a receiver may employ any in¬ 
termediate number of stages, depending 
upon the purpose for which it is intended 
and the means of the constructor. 

For the convenience of those who 
wish to make their own sets, the Radio Guild has devised a unit 




AMPU-UNITS 117 

system of amplifiers known as “Ampli-Units” which permit any 
combination of radio or audio frequency amplification to be easily 
used in or added to any receiver. Fig. 107 shows a one-stage 
audio frequency amplifying unit; this photograph portrays the 
principles underlying the design of these units. The necessary 



apparatus is condensed into a very small space, thereby greatly 
improving the efficiency as the wiring to the transformer and 
other apparatus is made with very short leads. Fig. 108 shows a 
detector and one-stage audio unit and Fig. 109 a detector and 
two-stage audio unit. Other combinations of radio as well as 
audio frequency amplification are made in a similar manner. In 
Fig. 110, a two-stage radio, detector and one-stage audio unit is 



shown with a two-stage audio unit. These “Ampli-Units” are 
completely self-contained and they can be obtained completely 
wired and ready for use. All apparatus is firmly attached to the 
horizontal panel of formica. Any unit can be used in or added 
to a receiver by drilling a few holes in the front panel of the set 
and then screwing the unit in place with a screw driver. Nd 
other tools are necessary. The unit is connected in the circuit 
bv wiring to the binding posts on its panel. 

However being self-contained, it is not even necessary to 
use a front panel with the units if it is not convenient or de¬ 
sirable to do so. 









118 


CONSTRUCTION OF RECEIVERS 

THE R.G. 510 AMPLI-UNIT. 

The R.G. 510 Ampli-Unit (so called because it is employed 
in the complete R.G. 510 receiver describer in the next chapter) 
represents a splendid adaptation of the unit system tt> a cascade 
radio and audio frequency amplifier of very high efficiency. In 
addition to being compact, the unit is designed to minimize plate 
and grid connections in both radio and audio frequency ampli¬ 
fying circuits. The complete unit is shown in Fig. 121. 

The Circuit: The wiring diagram of the R.G. 510 unit is 
given in Fig. 111. The unit has two stages of DX transformer- 
coupled radio frequency amplification, detector and three stages 
of audio frequency amplification. While the wiring diagram has 



a more complicated appearance than the circuits of radio and 
audio frequency amplifiers given in Part 1, the complications 
are caused by the filament wiring to the telephone jacks. The 
telephone jacks are arranged so that the ’phones can be plugged 
in the first, second, or third stage of audio frequency amplifi¬ 
cation, as desired. The jacks are of the filament control type 
so that only the filaments of the tubes in use are lighted when 
the plug is inserted in the jack. Otherwise, the circuit conforms 
exactly with the simple circuits of transformer-coupled radio and 
audio frequency amplifiers explained in Part 1. A potentiometer 
is used to prevent self-oscillation in the radio frequency amplifier. 

The input binding-posts are arranged so that the unit may 
be used with a tuner in a complete antenna receiving set or 
with a loop as antenna. There are four binding-posts and loop 
jack to the left of the unit which appear at the upper left-hand 
corner of the diagram of Fig. 111. A variable condenser is 
connected to two of these binding posts and the condenser is 
used to tune either the loop circuit (when a loop is plugged in 
the jack) or the secondary circuit of the aerial tuner (when 
the loop is removed from the jack). The secondary induc¬ 
tance of the tuner is connected to the two “tuner” terminals 
















































































AMPLI-UNITS 


119 


on the unit. This system of combining aerial or loop recep¬ 
tion in one receiver was explained in Paragraph 556 of Part 1. 

Apparatus Used to Construct Unit: The following apparatus 
is used in the construction of this unit: 

1 Formica panel measuring 5 x 17 x inches. 

6 Tube sockets. 

1 Potentiometer. 

2 Filkostats. 

3 Amperites and Mountings. 

1 Grid Condenser (.00025 mfd). 

1 Grid Leak (1 or 2 megohm). 

1 Double Circuit Loop Jack. 

3 Filament Control Telephone Jacks (1 Single Circuit, 2 
Double Circuit). 

2 DX Radio Frequency Transformers. 

8 DX Transformer Mounting Lugs. 

3 Guild Seal Audio Frequency Transformers. 

2 Fixed Condenser (.001 mfd) and 1 Fixed Condenser (.002 
mfd). 

9 Binding-posts. 

Sundry screws, brackets and wire. 

The panel to which all the apparatus in the unit is firmly 
attached must necessarily be strong and at the same time have 
good insulating properties. One-fourth inch Formica is there¬ 
fore used for this panel. The strength and insu¬ 
lating qualities of this material are well known. 

Special attention is called to the filament 
controls employed in the unit. To regulate the 
filament temperature of the radio frequency am¬ 
plifying tubes and the detector tube, vernier fila¬ 
ment controls of the Filkostat type, shown in Fig. 

112, are used. These provide exceedingly fine 
regulation of the filament temperature, which is 
very essential if maximum efficiency is to be ob¬ 
tained. The Filkostats are particularly useful 
when tuning in a weak station. A very close 
adjustment of the filament temperature is often 
the easiest method of controlling the sensitive¬ 
ness of the radio frequency amplifying circuit. 

An ordinary wire rheostat is useless for this pur- pi g% \\2 
pose but the Filkostat gives such a very fine con¬ 
trol of the filament temperature that it has become an almost in¬ 
dispensable adjunct of the radio frequency amplifier. 

The filaments of the audio frequency amplifying tubes do 
not require such careful adjustment and to minimize the num¬ 
ber or controls automatic self-adjusting resistances as shown 
in Fig. 113 are used in place of variable rheostats. These “Am- 
perities” have proved very satisfactory in operation. They tend 
to keep the filament current constant at a value suited to the 
tube in use. The Amperites are made in different models for 
different types of tubes. They are inserted in mountings so that 




120 


CONSTRUCTION OF RECEIVERS 


the Amperite can be suited to the tube employed. For instance, 
if a U.V. 201A tube is used with a six-volt filament battery, 

an Amperite “IA” gives the 
proper control of filament cur¬ 
rent. 

Filament control jacks, as 
used in the R.G. 510 Ampli- 
Unit are exceedingly useful in 
the audio frequency amplifying 
stages of a radio receiver. 
They simplify the operation and prevent the possibility of leav¬ 
ing the tubes alight while the set is not in use. It has been diffi¬ 
cult, however, to locate a jack which is properly designed to 
withstand the wear and tear of constant use. We have chosen 
the jacks shown in Fig. 114 and they have given exceptional 
service. Receivers using these 
jacks have been in daily use for 
many months and the jacks have 
never failed to operate perfectly. 

The main reason for this lies in 
the design and construction; the 
upper spring is reinforced so that the jacks never lose their 
springiness. 

The DX radio frequency transformers employed in the units 
were fully described in Part 1, Lesson 7. The audio frequency 
transformers were also described in the same lesson. Both types 
of transformers are essential parts of the unit and must be used 
to gain satisfactory results. 

Dubilier fixed condensers are used 
in the unit. These condensers with 
their mica dialectric are the standard 
of the radio industry. Fig. 115 shows 
the grid condenser with its convenient 
grid leak mounting. 

All the other apparatus in the unit, 
down to the last screw, is the finest 
that can be made or purchased. A 
radio frequency amplifier must be 
made with the best of material or its efficiency is greatly im¬ 
paired. 

Assembling the Unit: The first work of construction is the 
drilling of the panel to which the apparatus is attached. A 
scale drawing of the unit panel is given in Fig. 116 which shows 
the location of all holes drilled for the mounting screws and 
tube sockets. It is well to grain the panel on top and bottom 
after all the holes have been drilled. 

With the possible exception of the tube sockets, the mount¬ 
ing of the apparatus on the panel is fairly simple. Figs. 117 
and 118 show top and bottom views respectively of the com¬ 
pletely mounted but unwired unit and clearly reveal the loca¬ 
tion of the different pieces of apparatus. The tube sockets are 
spun into the Formica by a special machine operation which 
cannot be duplicated by amateur devices. This, however, will 




Fig. 114 












AMPLI-UNITS 


121 


not be a hindrance.as the units may be purchased in any stage 
of construction. The contact springs of the tube sockets are 
firmly secured to the lower side of the panels. 



Fig. 116 


In Fig. 117, between the first and second and the second and 
third tube sockets, may be seen the mountings. for the DX 
transformers. To the extreme left the loop jack is visible, to- 



Fig. 118 

It will be noted in Fig. 118 that the first audio frequency 
transformer is mounted directly beneath the first audio fre¬ 
quency tube socket. The second transformer is mounted to the 
rear of the second socket and the last transformer under the 
third socket. This staggered arrangement allows more space 
between the transformers and tends to prevent interaction be¬ 
tween the circuits, which might produce howling. 

Notes on Wiring: The photograph of Fig. 119 shows a 
lower view of the unit completely wired. With the exception 
of the audio frequency transformer leads, all wiring is made 












122 


CONSTRUCTION OF RECEIVERS 


with square bus-bar, combining neatness and strength with ef¬ 
ficiency. All wiring is underneath the panel. Although the 
circuit is more or less complicated, this mode of wiring tends 
to simplicity. All leads are made as short as possible and all 



Fig. 119 


joints soldered, not in a haphazard fashion but in such a man¬ 
ner that both electrically and mechanically the joints are per¬ 
fect. 

The leads from the audio frequency transformers are made 
with the flexible wire of the transformer coils themselves cov¬ 
ered with the insulating tubing. Fig. 85 of Part 1 clearly shows 



how these leads are made. As the transformers are mounted 
directly beneath the tube sockets, the wiring to the grid and 
plate terminals is exceptionally short and direct. The trans¬ 
formers are wired so that the outside of the primary coil con¬ 
nects to the plate of one tube and the outside of the secondary 
coil to the grid of the succeeding tube, as illustrated by the 
sketch of Fig. 120. If this sketch is not followed, howling may 
be caused. 

Fig. 121 is a close-up of the wiring from the condenser 
and tuner terminals to the loop jack. The wiring must be made 
in this manner to conform with the engraving of the terminals 
on top of the panel. When connections are made to these ter¬ 
minals in accordance with the engraved instructions, the “tuner” 
terminals are isolated from the remainder of the circuit if a 
loop is plugged into the double circuit jack. Additionally, when 
connecting the variable condenser to the marked terminals, 
the movable plates may be connected to the “F” or filament 













AMPU-UNITS 123 

terminal so that body capacity effects are avoided when operat¬ 
ing the receiver. 



Fig. 121 

INSTALLATION AND OPERATION. 

The R.G. 510 unit is principally intended to be used in 
the construction of a complete receiver, as described in the 
next lesson. However, with the addition of a tuning con¬ 
denser and loop antenna, the unit is a complete receiving set in 
itself, as suggested by the photograph of Fig. 122. Incidentally, 
this photograph shows the extreme compactness of the unit 
and shows how the radio frequency transformers are mounted, 
and the arrangement of amperites, potentiometer, Filkostats, 
valves, etc. 

A variable condenser is shown connected to the “condenser” 
terminals on the unit and a loop is plugged into the loop jack. 
The jack acts as a support for the loop. The condenser has a 
separate “vernier” plate which is of assistance when tuning the 
loop circuit to resonance. 

To install and operate this unit as a complete loop receiver 
the following procedure should be observed. 

Installation: 1. Make all battery connections exactly as in 
Fig 123. As shown in this photograph, a 60-80- (or larger) 
ampere hour six volt storage “A” battery and two forty-five 
volt plate batteries are required. Connect batteries to the ter¬ 
minals with heavy, insulated, flexible wire. Make sure that 
both A and B batteries are in perfect condition. 

2. Insert the radio frequency transformers, the grid leak 
(usually one megohm) and the three Amperites in their respec¬ 
tive mountings (Type “1A” Amperite for ^ Amp. and type 
“PT.” for 1 Amp. tubes). 

3. Connect a 23 plate vernier variable condenser to the 
“Condenser” terminals with the rotary plates to the terminal 



124 


CONSTRUCTION OF RECEIVERS 


marked “F.” Insert the loop plug in the loop jack, making sure 
that positive contact is made. 

4. Place the vacuum tubes in their respective sockets. Al¬ 
most any type of vacuum tube may be used for either the radio 



or audio frequency amplifier. The best results are obtained 
with De Forest or similar low internal capacity tubes in the 
radio frequency amplifier. A U.V. 200 (or C300) tube may be 
used as detector although practically as good results are had 
with the 201A type. 

Operation: 1. Insert the telephones or loud speaker plug 
in the center phone jack. The filaments of the tubes will auto¬ 
matically light, except the last audio frequency amplifier, which 
is then out of the circuit. Adjustment of the filaments of the 
radio frequency amplifying tubes is secured by manipulation of 
the Filkostat knob directly to the right of the potentiometer. 
Detector filament control is provided by the Filkostat under the 
detector tube. If a U.V. 200 or other gas content tube is used 






AMPLI-UNITS 


125 


for the detector the Filkostat should be turned until a hissing 
sound is heard in the telephones. The position of the Filkostat 
just below this “hissing point” gives best retification. 

2. The tuning condenser and the potentiometer are the im¬ 
portant operating controls. A few moments actual operation 
of the receiver will enable anyone to obtain satisfactory re¬ 
sults. The usual method of quickly tuning in a station is this : 
Turn the potentiometer to the negative side of the filament line 



Fig. 123 


and vary the tuning condenser between maximum and mini¬ 
mum until the carrier wave of a transmitting station is heard. 
The so-called carrier waves are discernible as two whistles on 
each side of a silent spot. When a broadcasting station is in 
action the modulated tones are heard between the two whistles. 
Having located the station, turn the potentiometer slowly in 
the opposite direction towards the positive side of the filament. 
At the same time make slight readjustments of the vernier of 
the tuning condenser to keep the station in tune. Then find 
the adjustment of the potentiometer with which the loudest sig¬ 
nals' are heard without the presence of the carrier wave. 

Having tuned in a station, any one of the three jacks may 
be used, depending on the volume of reproduction desired. The 
first jack is sufficient for head ’phones. The center jack is em¬ 
ployed for usual home entertainment with loud speaker, while 
by plugging in the last stage, very loud signals, suitable for 
a large room, are obtained. 

To pick up a different station, turn the potentiometer to 
the negative side of the filament and proceed as outlined above. 
The approximate condenser positions for the different stations 
should be recorded for future reference. 

The position of the loop is important and maximum signal 
strength is secured when the plane of the loop is pointed in 
the direction of the transmitting station to be received. The 
loop assists in eliminating interference from other stations which 
are not in the direction in which the loop is turned. 


126 


CONSTRUCTION OF RECEIVERS 


If a ground is connected to the “G” or grid terminal of the 
unit, the audibility of the system is greatly increased. Distant 
stations are easily received. When the ground is connected in 
this way the movable plates of the tuning condenser should also 
be connected to the grid post. 

Performance: There is no question that a six-tube receiver 
of this type is excellent for long distance reception and has 
proved its merits on innumerable occasions. The author, in his 
New York studio, has frequently picked up stations 1,500 miles 
distant, with equipment similar to that pictured in Fig. 122. 
Naturally, local reception is perfect and is usually accomplished 
on a small three-inch coil with no outside connection whatso¬ 
ever. For reliable reception of distant stations, however, it is 
much better to use an aerial with tuning arrangement as de¬ 
scribed in the next lesson. 


LESSON 10. 


RECEIVER TYPE R.G. 510. 


Complete Aerial or Loop Receiver with Tuner, 2-stage Radio 
Amplifier, Detector and 3-stage Audio Amplifier. 

There is seldom a radio amateur who is completely satisfied 
with his receiver. He starts, perhaps, with a crystal set which 
picks up the local stations. Soon he sells that and buys a new 
set that will receive farther. He hears stations two hundred 
miles away. But that isn’t far enough. He adds an amplifier— 
but still, there is something just a bit better if he could only 
find it. There is always a '‘perfect” receiver to dream about 
and perhaps one day possess. He knows exactly what that “per¬ 
fect” receiver must be able to do. It must be able to receive 
thousands of miles—on any wave-length. It must be absolutely 
selective—no interference. It must be simple to operate, etc., 
etc. Each individual has his own ideas as to what it should con¬ 
tain and it would be impossible to make a single receiver which 
would satisfy everybody. 

We are encouraged to learn from many radio amateurs, 
however, that the R.G. 510 receiver described in this lesson has 
even transcended their most fanciful dreams of a “perfect” re¬ 
ceiver. 

We are inclined to share the enthusiasm of those who are 
using this receiver. We have obtained quite phenomenal results 
with it under test conditions. Below are a few of the practical 
features of this instrument which combine to form as “perfect” 
a receiver as can be devised: 

Audibility: The R.G. 510 uses in its construction the unit 
described in the last lesson with two stages of radio frequency 
amplification, detector and three stages of audio frequency am¬ 
plification. The extreme sensitiveness of a receiver with this 
combination of amplification is apparent. The design and ap¬ 
paratus used in the amplifiers ensure maximum efficiency. Real 
amplification is obtained. One cannot judge the audibility of 
a receiver by the number of tubes that are used. The thing 
that counts is the amplification per stage. The high amplifica¬ 
tion of both types of amplifiers in the R.G. 510 unit was fully 
explained in Lesson 7 of Part 1. 

However, the audibility of any receiver must finally be 
judged under actual operating conditions. The results secured 
with the R.G. 510 under normal conditions indicate that the audi¬ 
bility is very high. Broadcasting stations over 1,000 miles dis¬ 
tant are consistently received, using only a small loop as antenna. 
With an outdoor aerial, the receiving range is doubled and some- 


128 


CONSTRUCTION OF RECEIVERS 


times trebled. These results, of course, do not represent the 
greatest distances that have been covered by this receiver; they 
merely represent the average. One amateur in Cuba, using loop 
antenna, reports the reception of a California broadcasting sta¬ 
tion. Another amateur in Cuba sends a list of U. S. broadcast¬ 
ing stations he has heard and the list is quite comprehensive. 
It includes Schenectady, New York, Boston, Massachusetts; 
Davenport, Iowa; Atlanta, Ga., etc., etc. Still another amateur 
in Minneapolis, Minn., reports hearing both the Atlantic and 
Pacific coast stations with great regularity. From all over the 
United States and Canada similar reports of long-distance re¬ 
ception have been made. 


There is a distinct fascination in possessing a receiver which 
is capable of picking up broadcasts from stations thousands of 
miles distant. Even though one rarely exercises this power, 
there is present a sensation of mastery in the knowledge that 
one can receive the distant stations at will. As an automobile 
manufacturer has declared, the buyer of an automobile prefer¬ 
ably chooses the high-power car—the car which is capable of 
making a speed of 80 or 90 miles an hour. He may never drive 
it at a greater speed than 40 or 50 miles an hour, but when he 
owns the high-power car he has the “consciousness of the pos¬ 
session of power’—the knowledge that, if put to it, he can make 
a higher speed. Similarly, the buyer of a radio receiver chooses 
the set with high audibility—the set that can receive stations 
thousands of miles away. 

Apart from the sensations of the owner, however, the buyer 
of the high-power car or the purchaser of a receiving set with 
high audibility, unconsciously makes a wise choice. One should 
rarely drive a car or operate a radio receiver with the maximum 
expenditure of power. If a car is invariably driven at the maxi¬ 
mum speed of which it is capable, it will soon wear out. Some 
power should always be held in reserve. Similarly, a radio re¬ 
ceiver should seldom be operated at the point of maximum sensi¬ 
tiveness ; the set will not wear out any quicker but distortion 
is much more likely to be experienced when the set is balanced 
at the critical point of maximum sensitivity. The receiver should 
preferably have such high audibility that it is not necessary to 
bring it to the. state of .maximum sensitiveness to receive local 
stations or stations within a radius of say, 1,000 miles The R G 
510 is a receiver of this type. To reproduce the broadcasting 
of the local stations or stations within a limited radius it is 
not necessary to operate the receiver with maximum output of 
energy The receiver does not need to be critically adjusted 
to produce the maximum amplification of which it is capable 
A rough adjustment serves the purpose and some amplification 
is . held in reserve.” However, the high audibility of the re¬ 
ceiver is there to be called upon if desired. By carefully ad¬ 
justing the radio frequency, amplifier, the tuner controls, and 
using all three stages of audio frequency amplification, the verv 
far distant stations can be brought in loudly and clearly at will. 
, . Selectivity:. As we ex P lain ed in Eesson 8, a receiver with 
high audibility is useless, unless, at the same time, it has high 


RECEIVER R. G. 510 


129 


selectivity. The practicability of radio frequency amplification 
with an outdoor aerial has often been questioned by experi¬ 
menters who have found the combination exceptionally broad in 
tuning. As consistent long-distance reception is only practical 
when some form of aerial is employed, this lack of selectivity 
constituted a severe objection to the use of radio frequency am¬ 
plification. To gain selectivity, many amateurs resorted to the 
use of loop antennae but, as we explained previously, the selec¬ 
tivity of loop reception is chiefly gained by enormously decreas¬ 
ing the audibility of the receiver. 

The efficient use of radio frequency amplification with an 
outdoor aerial is now made possible, however, by a special coup¬ 
ler which is described in Lesson 8. 

Go back and read Lesson 8 again. It explains how this 
“Harkness Coupler,” which is used in the R.G. 510 receiver, 
accurately controls the audibility and selectivity of the sys¬ 
tem; how it makes possible the reception of local or distant 
stations with a remarkable freedom from interference. 

Simplicity: When it is said that a receiver is “simple to 
operate,” some amateurs immediately conclude that the receiver 
must be inefficient. A common but entirely erroneous concep¬ 
tion of an efficient receiver is one with about fifteen controls. 
But simplicity of operation is as much a factor of efficiency 
as audibility or selectivity. If a receiver has seven or eight con¬ 
trols and each control requires careful adjustment, the receiver 
is absolutely useless for practical purposes no matter how high 
the audibility or selectivity may be. 

The R.G. 510, when receiving with the loop, has only two 
active controls. One control tunes the loop circuit and the 
other acts as a stabilizer. When receiving with the aerial there 
are four active controls; but tuning is not complicated. Of the 
four controls, two are mainly used for tuning the set to receive 
different stations. The remaining two controls are additional 
refinements for improving the audibility or selectivity of a sig¬ 
nal when it has been located by the two main controls. 

Other features: The R.G. 510 can be easily adapted to re¬ 
ceive on any wave-length—from 200 meters to 20,000 meters. 
This is a decidedly unique feature for a receiver of this type 
employing radio frequency amplification. 

Either a loop or outside aerial can be used to pick up sig¬ 
nals. The receiving range with the aerial, of course, is much 
greater than with the loop; that is to say, the audibility is 
much higher. Moreover, reception with the aerial is even more 
selective than with the loop. Therefore, although provision is 
made for using a loop, this should only be resorted to when it is 
inconvenient to erect an aerial. 

One, two, or three stages of audio frequency amplification 
can be used as desired by simply plugging in the telephones or 
loud-speaker in the proper jack. The jacks are of the filament 
control type. When the 'phone plug is removed from any jack, 
the filaments of all tubes are automatically switched off. When 
the ’phone plug is inserted in any jack, the filaments of the tubes 
in use are automatically lit; tubes not in use do not light. 


130 


CONSTRUCTION OF RECEIVERS 


The Circuit: The diagram of Fig. 124 shows the tuner of 
the R.G. 510 connecting to the amplifying unit (described in 
Lesson 9) which constitutes the remainder of the receiver. The 
wiring diagram of the unit itself was given in Fig. 111. 



tenna circuit is broadly tuned by varying the primary inductance 
of the coupler and is accurately tuned to resonance with a vari¬ 
able condenser in series with the ground lead. The secondary 
circuit is formed by the rotor of the Harkness Coupler and the 
secondary variable condenser. The inductive coupling between 
the two circuits is varied by revolving the rotor of the coupler. 

If a loop is inserted in the jack on the amplifying unit, 
the rotor of the Harkness Coupler is disconnected and the sec¬ 
ondary variable condenser is connected across the loop. The 
secondary condenser is then used to tune the loop circuit. 

CONSTRUCTING THE R.G. 510. 

Apparatus Used: The following apparatus is used in the 
construction of the R.G. 510: 

1 Formica panel measuring 1014" x 26" x 3/16". 

1 Harkness Coupler. 

1 Inductance Switch set (including lever, 4 switch points 

and 2 switch stops). 

2 Variable Condensers (.0005 mfd each). 

1 R.G. 510 six tube Ampli-Unit (completely wired). 

6 Binding Posts. 

3 Dials. 

1 Cabinet measuring 10^"x26"x9" (outside measure¬ 
ments). Sundry screws and wire. 















RECEIVER R. G. 510 


131 


Of the above apparatus, the six tube Ampli-Unit was fully 
described in the last lesson and the Harkness Coupler was de¬ 
scribed in Lesson 8 of Part 1. 

The remaining apparatus is all standard. Attention is 
drawn to the variable condensers, one of which is shown in the 
photograph of Fig. 125. These condensers 
are exceedingly well made, have an excel¬ 
lent appearance and are designed properly 
to eliminate electrical losses. A pig-tail 
contact is used from the rotary plates. 

Drilling of Panel: The first operation 
in the construction of the R.G. 510 is the 
drilling of the front panel. A scale draw¬ 
ing of this panel is given in Fig. 126 which 
shows the location of holes drilled for the 
mounting of the apparatus. The panel is 
grained and engraved after the holes are 
drilled. 

Assembly and Wiring: The use of the 
completely wired amplifying unit in the 
construction of the R.G. 510 greatly simpli¬ 
fies the work of assembly and wiring. A Fig. 125 

soldering iron and a screw driver are the 

only tools required to complete the work. By following the in¬ 
structions given below and with the assistance of the photo- 



( Boosts'*) 


Ant BPost 


< 5 > 

♦ Harkrwss 

<* : coup,e ' 

Switch’ « 


# © # 


& 

Gnd Primary 

Bhosl Condenser 


Secondary 

ConSenser 


o o o 
o o o 
o o o 
o o o 




o- 


Potentiometer 


^ Fittestau 


O' 

/ 


o o o 
Ci o o 
o o o 
o o o 


-o 


V 


l / 




Fig. 126 

graphic illustrations, any amateur constructor can easily put 
this set together and completely wire it in about half an hour. 

1. Mount the Harkness Coupler, the two variable con¬ 
densers, the inductance switch set and the six binding posts on 
the panel in the manner indicated in Fig. 127. # # 

2 Completely wire the antenna circuit. Follow the wiring 
diagram of Fig. 124. The photograph of Fig. 127 also shows the 
method of wiring very clearly. Make the leads from the coupler 
taps to the switch points with flexible wire covered by cambric 
tubing. Make all other wiring with bus bar. Also connect a 
wire between one of the Fahnestock clips (secondary terminals) 
on the coupler and one of the secondary loading coil binding 
posts as shown in Fig, 127. 








132 


CONSTRUCTION OF RECEIVERS 


3. Remove the potentiometer and Filkostat knobs and the 
nuts of the telephone jacks from the amplifying unit. Then 
mount the unit in place on the back of the panel, as illustrated 
in Fig. 128. 



4. Firmly secure the unit in place by screwing on the nuts 
of the telephone jacks projecting through the front of the panel 
and by fastening the potentiometer and Filkostats to the front 
panel with nuts and bolts. Fig. 129 shows a constructor in the 
act of tightening down on the last mounting bolt of the potentio- 



Fig. 128 

meter, the mounting bolts of the Filkostats having already been 
fastened and the telephone jack nuts screwed on tightly. These 
mounting bolts and nuts easily hold the entire amplifying unit 
firmly in place and eliminate the necessity of brackets. This 
completes the assembly of the R.G. 510 receiver. 

5. To finish the work of construction, make wire connec¬ 
tions from the tuner to the binding post on the amplifying unit, 


RECEIVER R. G. 510 


133 


Fig. 130 shows very clearly how the connections are made. It 
should be noted that the movable plates of the secondary variable 



Fig. 129 



Fig. 130 



Main Control N 

Fig. 131 


condenser are connected to the “F” or filament side of the “con¬ 
denser” terminals on the unit. The object of this is to avoid 
body capacity effects when operating the receiver. 





134 CONSTRUCTION OF RECEIVERS 



6. The assembly and wiring completed, mount the three 
dials on the shafts of the variable condensers and Harkness 
Coupler projecting through to the front of the panel; also mount 
the potentiometer knob and tjie two Filkostat knobs. Then 
screw the finished set into its cabinet. Fig. 131 and Fig. 132 
give two different views of the complete receiver. 

If the above instructions are followed, the work of con¬ 
struction will be found exceedingly simple. As the amplifying 







RECEIVER R. G. 510 


135 


unit can be obtained completely wired and tested, perfect operat¬ 
ing results with the home-constructed set are assured. 

INSTALLATION AND OPERATION. 

Installation: To install the R.G. 510 receiver, proceed as 
follows: 

1. Shunt the primary and secondary loading coil terminals 
with short sections of bus bar and connect the five battery ter¬ 
minals to both A and B batteries, as indicated in Fig. 123 of the 
previous lesson. Make certain that these batteries are in good 
condition as otherwise satisfactory operation is not possible. 

2. Insert the vacuum tubes in the tube sockets and make 
sure that all the tubes are making positive contact with the 
socket springs. In the previous lesson, we gave particulars of 
the proper types of tubes to use with the R.G. 510. Insert the 
radio frequency transformers, grid leak and Amperites in their 
respective mountings. 

3. Connect the aerial and ground to the antenna and ground 
posts on the front of the panel. 

Operation: To receive with the loop, insert the loop through 
the hole in the cover of the cabinet and plug it into the jack 
on the unit panel. The aerial tuner is then isolated from the am¬ 
plifying unit with the exception of the secondary variable con¬ 
denser, which is used to tune the loop circuit. Loop reception is 
effected in exactly the same manner as described in Lesson 9. 

To receive with the aerial, remove the loop entirely. The 
tuning is almost as simple as when receiving with the loop. The 
secondary condenser and the potentiometer are the main con¬ 
trols. A rough adjustment of the antenna circuit is sufficient 
to pick up signals which are tuned in by these two main con¬ 
trols. When the signal has been located, a close adjustment of 
the antenna circuit can be made by revolving the antenna con¬ 
denser to improve the audibility. Similarly, the coupling can be 
varied by revolving the rotor of the coupler to improve the se¬ 
lectivity or audibility of the system. After a little experience 
anyone can easily secure maximum efficiency from the receiver. 
To pick up signals, the following brief outline of the procedure 
should be observed: 

1. Adjust the radio frequency and detector Filkostat fila¬ 
ment controls to the proper points for the vacuum tubes used. 

2. Set the coupling dial at maximum; the primary induc¬ 
tance switch at the bottom tap to include all of the coil in the 
antenna circuit and set the primary condenser at an approxi¬ 
mately half-way position. 

3. With the potentiometer slider at the negative side of the 
filament, rotate the secondary condenser slowly until a station 
or carrier whistle is heard; keep the condenser centered about 
this point and move the potentiometer slider gradually towards 
the positive side of the filament, making slight readjustments of 
the secondary condenser to keep the station in tune. 

4. Turn the coupling dial to about “50” and then revolve 
the primary condenser to tune the antenna circuit and thereby 
improve the audibility of the signal. These adjustments will 


136 


CONSTRUCTION OF RECEIVERS 


also necessitate slight readjustments of the secondary condenser. 

If interference is experienced, turn the coupling dial a few 
degrees toward zero; that is to say, loosen the coupling between 
the primary and secondary circuits, then retune both the antenna 
and secondary circuits. Repeat this process if necessary until the 
interfering signal is eliminated. Variation of the potentiometer 
setting also assists in securing selectivity. It is advisable to 
keep a record of the best positions of the antenna condenser, 
coupling dial and secondary condenser for the most common 
transmitting stations. 

The Filkostat filament controls will be found particularly 
useful when tuning the receiver for weak signals. When the 
position of the potentiometer has been found which gives 
maximum audibility of a weak signal, the audibility can often 
be greatly increased by slightly changing the temperature of the 
filaments of the r.f. amplifying tubes. A wire rheostat does not 
give a fine enough control of the filament current to make this 
useful adjustment but the Filkostat varies the filament current 
very gradually and evenly and therefore acts as a very valuable 
tuning control. 


LESSON 11. 


RADIO AMPLIFIER TYPE R.G. 500. 


Tuner and Two-Stage Radio Frequency Amplifier for Increasing 
the Range of Any Receiver. 

There are probably many readers of this book who appreci¬ 
ate the many advantages offered by the R.G. 510 receiver de¬ 
scribed in the previous lesson and who would like to possess 
such a receiver; but they may already have a radio receiver or 
a detector and audio frequency amplifier. The question which 
then presents itself to these amateurs has been asked by hun¬ 
dreds of others in a similar situation—“How can I add radio 
frequency amplification to my receiver ?” 

This question has been answered in a variety of ways but 
most of the suggestions offered necessitate changing the wiring 
or design of the receiver to which radio frequency amplifica¬ 
tion is to be added. Incidentally, many of the suggestions of¬ 
fered are exceedingly inefficient as no provision is made for in¬ 
creasing the selectivity of the system, although the audibility 
is enormously increased by adding the radio frequency amplifier. 

The highly efficient tuner and two-stage radio frequency 
amplifier described in this lesson can be used with any radio 
receiver (or detector and audio frequency amplifier) without 
changing any wiring or altering the receiver in any way. In 
other words, the owner of either a regenerative or a non-regen- 
erative receiver can add a thousand miles or more to his re¬ 
ceiving range; secure exceedingly selective reception and obtain 
the many other desirable features of the R.G. 510, by construct¬ 
ing at low cost the tuner and amplifier set forth in this lesson 
and using it in connection with his receiver. 

This tuner and amplifier (known as Type R.G. 500) can be 
used with any radio receiver or detecting system. When used 
in connection with a receiver and a two or three-stagq audio 
frequency amplifier, the remarkable audibility and selectivity 
of the R.G. 510 receiver are exactly duplicated. One of the 
most unique features of the R.G. 500 tuner and amplifier is the 
key switch with which the radio frequency amplifier may be cut 
out of the circuit if desired. This switch connects the tuner of 
the R.G. 500 directly to the receiver or detecting system with 
which it is used and opens the filament circuit of the radio fre¬ 
quency amplifier. This is a distinct advantage as the radio 
frequency amplifier is seldom required for local reception but 
is ready for action at the flick of a switch. 


138 


CONSTRUCTION OF RECEIVERS 


The Circuit: The diagram of Fig. 133 shows the complete 
circuit of the R.G. 500 tuner and amplifier. The tuner con¬ 
sists of a Harkness Coupler with two variable condensers and 



is exactly similar to the tuner of the R.G. 510. The radio fre¬ 
quency amplifier comprises two stages of DX transformer- 
coupled amplification. A DX transformer is used to couple the 
output of the amplifier to the receiver or detecting system. A 
double circuit jack is provided so that either a loop or an out¬ 
side aerial can be used to pick up signals. 

CONSTRUCTING THE R.G. 500. 

The R.G. 500 is divided into two main parts: the tuner and 
the radio frequency amplifier. The latter is constructed as a 
separate unit, which we will first describe in detail. 

The Amplifying Unit: The following apparatus is used in 
the construction of the amplifying unit: 

1 Formica panel measuring 5 x 6^4 x J4 inches. 

2 Tube Sockets. 

1 Potentiometer. 

. 1 Filkostat. 

1 Double Circuit Loop Jack. 

1 Anti-Capacitv Key Switch. 

2 DX Radio Frequency Transformers. 

8 DX Transformer Mounting Lugs. 

9 Binding Posts and 1 Fixed Condenser (.002 mfd). 

Sundry screws, brackets and wire. 

As the R.G. 500 amplifying unit is one of the system of 
“Ampli-Units” described in Lesson 9, the same high quality type 
of apparatus is used in its construction. This is the only unit, 
however, in which a key switch is used. A Federal anti-ca¬ 
pacity switch was chosen and has been found very satisfactory 
in operation. 

Assembling the Unit: The construction of this unit is very 
similar to that of the other units. The Formica panel to which 
all the apparatus is attached is first laid out in accordance with 



































RADIO AMPLIFIER R. G. 500 


139 


the scale drawing of Fig. 134. All holes shown in this diagram 
are accurately drilled and both sides of the panel are grained. 


6/f 



The sockets are spun in and all the other apparatus is firmly 
mounted to the panel with 6/32 machine screws. The photo¬ 



graphs (Figs. 135 and 136) show upper and lower views of the 
completely mounted but unwired unit. 










140 


CONSTRUCTION OF RECEIVERS 


Wiring of Unit: A separate and complete wiring diagram 
of the unit itself is given in Fig. 137. The wiring is made with 



Fig. 136 

short direct leads in the usual manner. The design of the unit 
permits short grid and plate leads, which is highly desirable. 
Reference should be made to Fig. 121 for details of wiring 
to the loop jack. The importance of carefully soldering and then 
cleaning all joints is again emphasized. Each joint, after being 



carefully soldered, is wiped clean with a dry cloth, then brushed 
with alcohol to dissolve any remaining flux and finally rubbed 
spotlessly cleau with a fresh cloth. Many home-made receivers 
and, in fact, some commercial ones, lose a great deal of ef¬ 
ficiency by neglecting this necessary work. 

































RADIO AMPLIFIER R. G. 510 


141 


Assembly of the Complete R.G. 500: The following ap¬ 
paratus, in addition to the completely wired amplifying unit just 
described, is necessary in constructing the R.G. 500: 

1 Formica panel measuring 10)4 x 17 x 3/16 inches. 

1 Harkness Coupler. 

1 Inductance Switch Set (including switch lever, four 

switch points and two switch stops). 

2 Variable Condensers (.0005 mfd each). 

8 Binding Posts. 

3 Dials. 

1 Cabinet, measuring 10)4 x 17 x 8)4 inches (outside). 

Sundry screws and wire. 

The above apparatus has already been fully described in 
previous lessons. 

The construction of the R.G. 500 from this point is very 
similar to the construction of the R.G. 510. The front panel 
is first drilled in accordance with the scale drawing of Fig. 138. 
The panel is grained and engraved after the holes are drilled. 




With the completely wired amplifying unit in hand, the 
assembly and wiring of the R.G. 500 are very simple. Again a 
soldering iron and a screw driver are the only tools required 
to complete the work. The below procedure should be followed: 

1. Mount the Harkness Coupler, the two variable con¬ 
densers, the inductance switch set and the eight binding posts 
on the panel in the manner indicated in Fig. 139. 

2 Completely wire the antenna circuit. Follow the wir¬ 
ing diagram of Fig. 133. The photograph of Fig. 139 shows how 
this wiring is made. Also connect a wire from one of the sec¬ 
ondary terminals of the coupler to one of the secondary load¬ 
ing coil binding posts. 

3. Remove the potentiometer, Filkostat and anti-capacity 
switch knobs; then mount the unit in place on the back of the 
panel as illustrated in Fig. 140. 


. Pri Load. 
/ B Posts * 


Ant BPost 


Switch 

Lever 


t 

Gnd 

BPost 


,Sec Load . 
I B Posts ) 
+ * 


* 


T v 

harkness 

Coupler 


Pn Condenser 


Sec Condenser 


<> 4 > + 

/ 


Potentiometer Filkostat 


* 
Output 
B Posts 

t $ + 

J U Switch 


<j> fy 


Fip. 381 








142 


CONSTRUCTION OF RECEIVERS 



4. Firmly secure the unit in place by screwing in the four 

mounting screws of the 
key switch, the two 
mounting bolts of the 
Filkostat and the two 
mounting bolts of the 
potentiometer. Brackets 
are unnecessary to hold 
the unit in place as 
these mounting screws 
and bolts hold the unit 
very securely to the 
front panel. 

5. Connect the 
tuner to the amplifying 
unit in exactly the same 
manner as previously 
shown in Fig. 130. 
Connect the movable 
plates of the secondary 
condenser to the “F” 
or filament side of the 
“condenser” terminals 
on the unit. Also wire 
the output terminals of 
the unit to the output binding'posts in the front panel. Fig. 141 
shows the rear view of the completely wired set. 


tig. 139 



ig. 140 

6. The assembly and wiring completed, mount the three 
dials on the shafts of the variable condensers and Harkness 


RADIO AMPLIFIER R. G. 500 


143 


Coupler; also mount the potentiometer knob, Filkostat knob and 
the key switch knob. Then screw the finished set into its cab¬ 
inet. Figs. 142 and 143 are front views of the complete R.G. 500. 



Fig. 141 

INSTALLATION OF THE R.G. 500. 

To install the R.G. 500 amplifier, proceed as follows: 

1. Shunt the primary and secondary loading coil terminals 
with short sections of wire. Connect a storage battery to the 

- “Fil” terminals on the amplifying unit and connect the positive 
terminal of a 90-volt plate battery to the “B Pos.” post on the 
amplifying unit. Connect the negative end of the plate battery 
to the positive side of the filament battery. Make certain that 
the batteries are in good condition. 

2. Insert the amplifying tubes in the tube sockets and make 
sure that the tubes are making positive contact with the socket 
springs. Insert the DX transformers in their mountings. 

3. Connect the aerial and ground to the antenna and ground 
posts on the front of the panel. 

The amplifier is then ready to be connected to a receiving set 
or simple detecting system. The R.G. 500 amplifier can be used 
with any of the following: 

1. A plain crystal rectifier. 

2. A vacuum tube rectifier. 

3. A simple tuner of any type and either a crystal or vacuum 
rectifier. 

One, two or three stages of audio frequency amplification 
may succeed the detecting system of any of the above three 
classes. 



144 


CONSTRUCTION OF RECEIVERS 


In connecting the amplifier to a detecting system under the 
first classification the “G” post of the output terminals is con¬ 
nected to one side of the crystal rectifier; the other side of the 



Fig. 142 


H 



Fig. 143 

crystal is connected to one telephone terminal and the remain¬ 
ing ’phone terminal is connected to the “F” post of the output 
terminals on the amplifier. If the crystal set contains a tuner 
of any description, the output posts of the amplifier are con¬ 
nected to the antenna and ground posts of the crystal receiver. 



With detecting systems of the second class, the output ter¬ 
minals of the amplifier are connected to the input posts of the 

















RADIO AMPLIFIER R. G. 500 


145. 


detector; that is to say, the output of the radio frequency ampli- 
fier is connected across the grid and filament of the rectifying 
tube. The filament and plate batteries of the radio frequency am¬ 
plifier can be used for the detector tube. Fig. 144 illustrates how 
the R.G. 500 amplifier is connected to a vacuum tube detecting 
system with audio frequency amplifier. Only one storage bat¬ 
tery and one 90-volt plate battery are required. 

If the amplifier is used with a simple tuner and detecting 
system in the third classification above, the output terminals 
of the amplifier are connected to the aerial and ground posts of 
the tuner while the battery connections are the same as de¬ 
scribed above. Fig. 143 illustrates how the amplifier is con¬ 
nected to a standard receiving set. 

When audio frequency amplification is used with any of 
the above systems, the connections remain the same. The bat¬ 
tery connections are as illustrated in Fig. 144. The use of audio 
frequency does not affect the tuning of a radio receiver in 
any way. 


OPERATION. 

The following general methods of reception are made con¬ 
veniently possible when the R.G. 500 is used with any type of 
receiver or detecting system: 

1. Loop reception with Radio Frequency Amplification. 

2. Aerial reception with Radio Frequency Amplification. 

3. Aerial reception without Radio Frequency Amplification. 

Loop reception without radio frequency amplification is also 

possible but is only practical when receiving nearby stations. 

When the anti-capacity key switch of the R.G. 500 is thrown 
to the down position, the circuit is changed (see Fig. 137), so 
that the secondary of the coupler is connected to the output ter¬ 
minal and the filament circuit of the amplifier is broken. When 
the switch is raised to the opposite position the secondary of the 
last radio frequency transformer is connected to the output ter¬ 
minal and the filament circuit of the amplifier is completed. 

Therefore, to use the radio frequency amplifier for either 
loop or aerial reception, throw the switch up; to cut off the am¬ 
plifier when receiving with the aerial, throw the switch down 
and slide the potentiometer to the positive side of the filament. 

If the R.G. 500 is used with either a plain crystal or vacuum 
tube rectifying system, the following operating instructions 
should be followed: 

For reception with either loop or aerial with radio frequency 
amplification (1 or 2 above), throw up the key switch to include 
the radio frequency amplifier in the circuit and then follow the in¬ 
structions for the operation of the R.G. 510 receiver given in Les¬ 
son 10. The operation of both receivers in this respect is identical. 

For reception with the aerial without radio frequency am¬ 
plification, remove the loop, switch off the radio frequency am¬ 
plifier and turn the potentiometer to the positive side of the fila¬ 
ment. If the detecting system has a crystal rectifier, connect a 
small fixed or variable condenser between the primary and sec- 


146 


CONSTRUCTION OF RECEIVERS 


ondary loading coil posts as shown in Fig. 143 and then manipu¬ 
late the three controls of the R.G. 500 tuner in accordance with 
the instructions given in previous lessons for tuning coupled cir¬ 
cuits. However, if the detecting system has a vacuum tube rec¬ 
tifier, as is common, the condenser shown in Fig. 143 is not re¬ 
quired, provided a variometer is used to tune the plate circuit of 
the vacuum tube detecting system. With the radio frequency 
ampifier switched off the R.G. 500 and a tuned plate detecting 
system together constitute a simple regenerative receiver ex¬ 
actly similar to the circuit of Fig. 71, Part 1. The antenna cir¬ 
cuit is tuned to resonance with an incoming signal by revolving 
the primary condenser of the R.G. 500. The secondary circuit is 
tuned to resonance with the secondary variable condenser, and 
the plate circuit is tuned with the plate variometer. The audi¬ 
bility and selectivity of the system is controlled by varying the 
coupling between the antenna and secondary circuits. A varia¬ 
tion of this coupling necessitates a slight readjustment of the 
primary and secondary condensers. When the R.G. 500 is used 
in this manner it will be found very helpful to use a Filkostat 
to control the filament current of the detector tube. This ver¬ 
nier adjustment of the detector filament current is very useful 
in controlling regeneration and greatly assists and simplifies 
the tuning. 

If the R.G. 500 is used with some standard type of receiving 
set which includes a simple tuning and detecting system, the 
operating instructions given below should be followed. In these 
instructions, the expression “receiver” refers to the standard re¬ 
ceiving set with which the R.G. 500 is used. 

For loop reception with radio frequency amplification, in¬ 
sert the loop in the loop jack and set the potentiometer to the 
negative side of the filament. Adjust the receiver controls to the 
approximate wave-length to be received and vary the secondary 
condenser of the R.G. 500 until the station is heard. Then vary 
the potentiometer and secondary condenser for better results. 
Finally tune the receiver accurately for maximum signal strength. 

For antenna reception with radio frequency amplification, 
tune the receiver in the manner described above; otherwise, fol¬ 
low exactly the operating instructions given in the last para¬ 
graphs of Lesson 10. 

For antenna reception without radio frequency amplification, 
either switch off the radio frequency amplifier or connect the 
aerial directly to the antenna post of the receiver with which 
the R.G. 500 is used. By the former method very selective tun¬ 
ing can be obtained but the latter method is preferable if the 
receiver already has a selective tuner; the tuning is less com¬ 
plicated and the audibility greater. 

LOADING COILS. 

It will be noted that loading coil terminals for the primary 
and secondary circuits of the tuner are provided in both the 
R.G. 500 and R.G. 510. For short wave reception, these ter¬ 
minals are normally shorted. To receive wave lengths over 600 
meters, the shorting lugs must be removed, loading coils of the 


RADIO AMPLIFIER R. G. 500 


147 


proper inductance value connected to the terminals and the radio 
frequency transformers changed. The loading coils may be laid 
on top of the cabinet or special mountings may be provided for 
them. A variable coupling is not essential. Usually they should 
be placed at right angles to each other. 

The following coils of the honeycomb, duo-lateral, or Gib- 
lin-Remler type may be used. The number is usually an indi¬ 
cation of the total number of turns on the particular coil: 

Primary Secondary 
600-1500 meters 150 100 

1500-3000 meters 300-400 300 

To cover this range of wave-lengths, the following types of 
DX radio frequency transformers should be inserted in the radio 
frequency amplifier: 

Type DXS 1 : 400-1200 meters. 

Type DX2: 900-3000 meters. 


LESSON 12. 


THE “NEUTRODYNE” SYSTEM 


Announcements have frequently been made that radio is 
about to be “Revolutionized.” Beyond the mere statement, 
nothing of a very startling nature seems to happen. From time 
to time these revolutionary expectations are heralded upon the 
publication of trick circuits which one is led to believe will im¬ 
mediately supersede all previous attempts at radio reception. 

But like ships that pass in the night, the revolutionary cir¬ 
cuits, which are usually old circuits with some doubtful varia¬ 
tions, create their little uproar and pass on in their way to 
oblivion. Unfortunately, however, in the avalanche of “new 
circuits” some perfectly good circuits are overlooked. The 
much maligned “super-regenerative” was cast aside because too 
much was expected of it. It was given so much publicity and 
received such rough handling on the part of many of its expon¬ 
ents that the amateur was led to believe that with the turn of 
a wrist he could receive China on one tube. When he failed to 
realize this expectation—or one just as fantastic—the circuit was 
discarded. 

Of late, the so-called “Neutrodyne” system has been given 



some publicity and the usual claim of revolutionizing radio has 
been made by some. The object of the system is to prevent 
self-oscillation in the circuits of a radio frequency amplifier. 
An example of the manner in which it accomplishes this is shown 
in the theoretical circuit diagram of Fig. 145. This circuit shows 






























THE NEUTRODYNE SYSTEM 


149 


two stages of radio frequency amplification arid detector. T2 
an d T3 are inter-tube coupling transformers. To counteract the 
feed-back effect of the internal capacity of the amplifying tubes 
in the circuit the grid circuit of the second tube is coupled back 
to the grid circuit of the first tube by the condenser Cl and the 
secondary of the transformer T3 is coupled back to the grid 
circuit of the second amplifying tube by the condenser C2. 

The author fails to see anything particularly “revolution¬ 
ary” . a b. ou t this circuit. By this he does not mean that the cir¬ 
cuit is inefficient; on the contrary, the principle of neutralizing 
the capacity feed-back of amplifying tubes in a radio frequency 
amplifier is a very good one. The circuit itself, with the excep¬ 
tion of the neutralizing condensers, is, of course, a standard 
transformer coupled radio frequency amplifying circuit. This 
will be evident if the circuit of Fig. 145 is compared with Fig. 
87 of Part 1. In paragraphs 454 to 459 we explained fully the 
operation of this circuit. The transformers Tl, T2 and T3 of 
Fig. 145 are constructed as described in these paragraphs; that 
is to say, the primary of each transformer is made aperiodic and 
each secondary circuit is tuned to resonance with a variable 
condenser. The circuit, then, is a modified tuned radio fre¬ 
quency amplifying circuit. 

As regards the neutralizing condensers Cl and C2, some 
original explanations have been given of the action of these con¬ 
densers but we are content to believe that the capacity feed¬ 
back of the amplifying tubes is neutralized by the negative feed¬ 
back produced by these condensers. The manner in which self¬ 
oscillation in a radio frequency amplifier is controlled by nega¬ 
tive feed-back action was fully explained in Paragraphs 479 to 
485 of Part 1. 

Fig. 146 shows a side view of an experimental “Neutrodyne” 
receiver constructed by the author. This set has two stages of 
radio frequency amplification, detector and one stage of audio. 
The published circuit and specifications of the Neutrodyne re¬ 
ceiver were followed. One of the radio frequency transformers 
is visible in the foreground between the detector tube and the 
audio frequency amplifying tube. Each of the three trans¬ 
formers has eight turns on the primary and fifty turns on the 
secondary. The coils are wound on 3 y 2 " tubing. The second¬ 
ary is laid over the primary and only separated from it by a 
small spacing. The two neutralizing condensers are visible 
along the back of the panel. The drawing of Fig. 147 shows 
how these neutralizing condensers are made. The capacity of 
each condenser, of course, is very small, being about equal to the 
capacity of the tubes in the circuit. Following specifications, 
the transformers were all turned at an angle of 56 degrees and 
supported behind the three variable condensers used for tuning 
the grid circuits. 

In operation, the receiver was found to oscillate if the neu¬ 
tralizing condensers were not adjusted properly. When the 
neutralizing condensers were adjusted and self-oscillation thereby 
prevented the selectivity of the system was excellent and the 
audibility fair. The audibility, of course, was not nearly so 


150 


CONSTRUCTION OF RECEIVERS 



good as with a receiver comprising two stages of DX trans¬ 
former-coupled radio frequency amplification, detector and one 
stage of audio; but this was to be expected, as the efficiency of 
the DX transformer is very much greater than the simple type 
of transformer used in this receiver. Even although the second¬ 
ary of each transformer is tuned accurately to resonance, this 


Fig. 146 


does not compensate for the loss of amplification caused by the 
aperiodic primary. This could be remedied, of course, by tuning 
the primary circuits but the tuning of the system would then be 
so complicated that it would be impracticable. 

We made numerous tests with this receiver in an endeavor 

4 space Spaghetti"tubing 

(?) Fig. 147 

to . improve its audibility. Different sizes of transformers were 
tried and the number of turns on the primary was changed. 
It.was found, however, that even with only six turns on the 
primary of each transformer, self-oscillation was produced unless 
the neutralizing condensers were properly adjusted. In view 
of the fact that, in a receiver of this type using transformers 
with aperiodic primaries, the most important source of feed¬ 
back—the feed-back through the internal capacity of the tubes_ 

is very small as compared with a DX transformer coupled ampli¬ 
fier (See Par. 493, Part 1), it seemed to the author that there 












THE NEUTRODYNE SYSTEM 


151 


must be inductive coupling between the transformers themselves 
to produce self-oscillation, especially with only six turns on the 
primaries of the transformers. If this proved to be the case 
the amplification of the system, of course, must be small. As 
we explained in Paragraph 490 of Part 1, all sources of coupling 
in a radio frequency amplifier must be reduced to an absolute 
minimum. If there is comparatively close inductive coupling 
between the transformers of a radio frequency amplifier, con¬ 
tinuous oscillations will be self-generated very easily. The 
only way of preventing these continuous oscillations is to re¬ 
duce the amplification of the system but the more easily the con¬ 
tinuous oscillations are generated the greater the amplification 
must be reduced to stop them. 



Fig. 148 


THE DOUBLE-DEE R. F. TRANSFORMER. 

Believing that the self-oscillation of the “Neutrodyne” re¬ 
ceiver was chiefly caused by inductive coupling between the 
radio frequency transformers the Author designed a special type 
of transformer to eliminate this inductive coupling. This unit 
is known as the “Double-Dee” radio frequency transformer and 
consists of a special air-core trans¬ 
former attached to the rear of a vari¬ 
able condenser as pictured in Fig. 148. 
It will be seen from this photograph 
that the transformer is composed of 
two sections shaped like the letter “D” 
and held together by bakelite rings. 
The coils of the transformer are wound 
on these two sections so that each 
holds one-half of the primary coil and 
one-half of the secondary coil, the pri- 
















152 


CONSTRUCTION OF RECEIVERS 


mary being wound over the secondary and separated from it by 
a piece of heavy insulating paper. The two halves of each coil 
are joined together so that the winding follows the direction sug¬ 
gested by Fig. 149. By arranging and winding the coils in this 
manner a closed magnetic field is created when current flows 
in the transformer. The magnetic field is so concentrated that 
the inductive coupling between two Double-Dee transformers, 
separated a few inches, is practically zero. 

Experiments prove that these Double-Dee transformers 
greatly improve the operation and stability of a tuned radio 
frequency amplifying receiver. The tendency towards self¬ 
oscillation caused by inductive coupling between transformers 
is eliminated and the Double-Dee transformer is designed so that 
the only remaining feed-back of importance—that produced by 
the capacitive coupling of the tubes—is not strong enough to 
generate continuous oscillations. The necessity for a potentio¬ 
meter or neutralizing condensers is obviated. Moreover, the am¬ 
plification of a receiver with Double-Dee transformers is higher 
than that of a receiver which uses transformers with ordinary 
solenoidal windings. The latter set “oscillates” when only 6 
turns of wire are used as the primary of each transformer 
whereas the former set does not generate continuous oscillations 
although 8 turns are used on the primary of each Double-Dee 
transformer. 

It may be mentioned that Double-Dee transformers are only 
intended for use in a receiver with two stages of radio frequency 
amplification. Ordinary transformers may be used in a receiver 
with only one stage of radio frequency amplification as inductive 
coupling can then be easily eliminated by merely turning the 
transformers at right angles to each other. 


HOW TO BUILD A 4-TUBE RECEIVER WITH DOUBLE- 
DEE TRANSFORMERS. 

A receiver using Double-Dee radio frequency transformers 
has many advantages. With high audibility and selectivity the 
system is easy to operate and comparatively inexpensive to con¬ 
struct. No troublesome adjustment of neutralizing condensers 
is required to prevent self-oscillation. In the remaining pages 
of this lesson we give particulars of a receiver of this type and 
sufficient constructional data to enable the reader to build the 
set if he so desires. 

The Circuit. The diagram of Fig. 150 shows the circuit used 
in this receiver (known as Type R.G. 515). This circuit com¬ 
prises a two stage modified tuned radio frequency amplifier, 
vacuum tube detector, one stage of reflex audio frequency ampli¬ 
fication and one extra stage of audio amplification. As the grid 
of each amplifying tube is connected directly to the negative 
side of the filament the reflex system can be used to advantage. 
This increases the audibility of the system without increasing 
the number of tubes. Filament control telephone jacks are used 


THE NEUTRODYNE SYSTEM 153 

to simplify the operation. The circuit is intended for use with 
U.V. 201A or C 301A tubes and Amperites are therefore em¬ 
ployed throughout to automatically control the filament current 
of the four tubes. This reduces the operating controls of the 



Fig. 150 

receiver to the three variable condensers for tuning the three 
grid circuits to resonance. 


CONSTRUCTION OF THE R.G. 515. 

For the benefit of those who would like to build a receiver 
of this type we give details of the construction of this receiver 
below: 

Apparatus Used: The following apparatus is used in the con¬ 
struction of the R.G. 515: 

1 Formica panel measuring 19X8X3/16 inches 

1 Formica panel measuring 11J4 X 7/4 X V\ inches 

4 tube sockets 

1 grid condenser (.00025 mfd) 

1 grid leak (1 megohm) 

2 Filament control jacks (1 single circuit & 1 double circuit) 

2 Radio Guild Audio Frequency Transformers 

4 Amperites and mountings 

7 Binding posts 

3 Double-Dee Radio Frequency Transformers 

3 Dials 

1 Cabinet, measuring 20y 2 X 8 X 8/2 inches outside mea¬ 
surements J 4 " material 

Sundry screws, brackets and wire. 

Assembling and Wiring: The front and rear panels are 
drilled in accordance with the scale drawings of Figs. 154 and 155. 
In assembling the receiver, the telephone jacks and the audio 
frequency transformers are attached to the lower side of the 
rear panel The tube sockets are spun in in the positions indi¬ 
cated The grid condenser is mounted on the top of the panel 
near the detector tube and the mountings for the four Amperites 



































































CONSTRUCTION OF RECEIVERS 


154 



Fig. 151 



Fig. 152 



Fig. 153 




























THE NEUTRODYNE SYSTEM 


155 


are also attached to the top of the rear panel. This panel is then 
wired separately, all the filament circuit and as much of the re¬ 
mainder of the circuit as possible being completed. The three 
Double-Dee transformers with their variable condensers are 
mounted on the front panel. Finally the shelf panel is mounted 
on the front panel with brass brackets and the wiring completed. 
Figs. 151 and 152 show two rear views of the receiver and Fig. 
153 shows the front view. 



Fig. 155 


Installation and Operation: To install the R.G. 515, insert 
four U V. 201A tubes in the tube sockets and connect a six volt 
storage battery and a 90 volt plate battery to the battery term¬ 
inals at the rear. Connect the antenna and ground to the bind¬ 
ing posts on the front panel. . 

The operation is then exceedingly simple. There are only 
the three controls and each circuit is tuned independently to 
resonance by these controls. The three dials can be calibrated 
in wave-length if desired. 


















LESSON 13. 

THE HARKNESS RECEIVER. 

Models A and B. 

To judge the efficiency of a radio receiver one must clearly 
understand the relative importance of the factors of efficiency 
upon which a judgment is based. Therefore, in order that the 
reader may easily judge and fully appreciate the advantages of 
the receivers which are described in this lesson we will briefly 
outline the relative importance of the essential qualities of a radio 
receiver, as graphically presented by Fig. 156. 

RELATIVE IMPORTANCE OF A RADIO RECEIVER’S 
QUALITIES. 

1. Selectivity: This factor is of prime importance. Above 
all things a radio receiver must be selective. The most useless 
set is one which cannot select and reproduce the broadcasting of 
a particular station to the exclusion of others. 

2. Audibility: This essential quality is second in impor¬ 
tance. For practical purposes a radio receiver must at least be 
sensitive enough to reproduce local stations on a loudspeaker. 
If the audibility be higher than this so that distant stations can 
also be reproduced on a loudspeaker, so much the better—pro¬ 
vided the selectivity is proportionately high. High audibility 
without high selectivity is useless. (See Par. 511.) 

3. Ease of Operation: This is a distinctly important qual¬ 
ity in a radio receiver and is often overlooked as being a factor 
in determining efficiency. If one type of receiver requires three 
controls but has no greater selectivity and audibility than an¬ 
other type with only two controls, the latter set is more efficient. 

4. Cost: The initial cost of a radio receiver is determined by 
the amount and quality of the apparatus required by the receiv¬ 
ing system employed. The additional cost of the accessories and 
upkeep cost plainly depend upon the number of vacuum tubes the 
system requires. It is evident that the receiving system which 
requires the least amount of apparatus and the smallest number of 
vacuum tubes to gain a given degree of selectivity and audibility 
(the quality of the apparatus in all cases being equal) is the low¬ 
est in cost and is therefore the most efficient. In case the quality 
of the apparatus is not equal and a given degree of audibility and 
selectivity is gained, on the one hand, by the use of a large quant¬ 
ity of poorly-made apparatus and, on the other hand, by the use 
of a small quantity of well-made apparatus the latter is more 
efficient, even though the actual cost of each receiver may be 
identical. The receiver which uses well-made apparatus will, of 


THE HARKNESS RECEIVER 157 

course, last longer, look better and will be less likely to develop 
faults than the other. 

5. Non-reradiation. This quality, while relatively unim¬ 
portant so far as the user of a radio receiver is concerned, is all¬ 
essential to the thousands of other owners of radio sets. Oscillat- 



vfd 

issential^ 
mportant^ 
Valuable — 


Hig'K Selectivity 
Hit'll Audibility 
Easy Operation j 

Cos t^^^| 



Fig. 156 


ing regenerative receivers are alone responsible for the whistles 
and squeals which accompany the reproduction of radio broad¬ 
casting by the more modern types of non-oscillating receivers. 
These whistles and squeals are most objectionable and entirely 
preventable. If the use of oscillating receivers for broadcast re¬ 
ception were prohibited the quality of radio reception would be 
vastly improved. 

THE HARKNESS RECEIVER. 

The present author believed a need existed for a non-oscil¬ 
lating radio receiver with high selectivity and high audibility 















158 


CONSTRUCTION OF RECEIVERS 


which would at the same time be inexpensive and easy to operate. 
After considerable experimenting he devised a circuit and de¬ 
signed a receiver which meets all these specifications in a sur¬ 
prising manner. The details of this system were initially pub¬ 
lished in the first edition of this work. The circuit has since be¬ 
come popular and is now generally known as the “Harkness 
Circuit”. Receivers using the circuit are known as “Harkness 
Receivers”. 

FUNDAMENTAL HARKNESS CIRCUIT. 

The fundamental Harkness Circuit is given in Fig. 157. The 
functioning of this circuit may be explained as follows: 

An incoming signal oscillation induces an oscillating e.m.f. 
across the secondary of the radio frequency transformer T2. The 
grid circuit is tuned to resonance by means of a variable con¬ 
denser and the signal oscillation applied between the grid and 
filament of the tube. The resulting radio frequency current vari¬ 
ations in the plate circuit set up oscillations across the primary 
of the second radio frequency transformer T2. An amplified 
oscillation is thereby induced in the secondary of T2 which is 
also tuned to resonance with a variable condenser. This magni¬ 
fied signal oscillation is then rectified by the crystal and the audio 
frequency variations of the rectified current induce an alternating 
e.m.f. across the secondary of the audio frequency transformer, 
the primary of which is included in the detecting circuit. The 
audio frequency variations are applied between the grid and fila¬ 
ment of the tube and the amplified variations in the plate circuit 
are detected by the telephones. The single tube amplifies .the 
radio and audio frequency variations simultaneously. 

Unlike most reflex receivers, the Harkness Receiver ampli¬ 
fies radio and audio frequency currents with full efficiency. It 
should be particularly noted that the grid of the reflex tube in the 
Harkness circuit is connected directly to the negative side of the 
filament. Those who have followed the contents of the previous 
lessons will realize the importance of this. The ordinary reflex 
circuit connects the grid of the amplifying tube to the center arm 
of a potentiometer which is shunted across the filament battery. 
The object of this, of course, is to control self-oscillation but 
while a potentiometer can be satisfactorily used for this purpose 
in a plain radio frequency amplifier its effect in a reflex circuit 
is very harmful. If the grid of a reflex amplifying tube is given 
a positive potential by a potentiometer the audio frequency ampli¬ 
fication is practically reduced to zero; the audio frequency trans¬ 
former might as well not be in the circuit at all and the set would 
probably operate a great deal better without it. (See Par. 437, 
Part 1.) 

QUALITIES OF THE HARKNESS RECEIVER. 

Audibility equal to 3-tube set: As the Harkness Receiver 
uses tuned radio frequency amplification and also amplifies with 
full efficiency at audio frequency—the grid of the single tube 
being connected directly to the negative side of the filament—the 
audibility of the receiver is consequently very high. The receiver 


THE HARKNESS RECEIVER 


159 


amplifies at radio and audio frequency just as though separate 
tubes were used for each type of amplification. In the fullest 
sense the single tube does the work of two tubes. Moreover, as 
a crystal is used as rectifier the audibility of the receiver is 
actually equal to that of a receiver with three tubes. Tests have 
proven this to be true. To severely test the audibility of the single 
tube Harkness Receiver, the author constructed a 3-tube set 
with one stage of tuned radio frequency amplification, a tube 
detector and one stage of audio frequency amplification. The 
audibility of this receiver was accurately compared with that of 



single tube Harkness Receiver and the difference between the two 
was extremely small. 

Operates Loudspeaker With One Tube: The audibility of 
the Harkness Receiver is so great that with only one tube it 
actually operates a loudspeaker with ease. With one tube the 
average range is 1000 to 1500 miles and hundreds of amateurs 
have testified that the stations within this range are often received 
with sufficient volume to operate a loudspeaker! 

Selectivity Very High: Since the Harkness Receiver uses 
tuned radio frequency amplification its high audibility is not 
gained at a sacrifice of selectivity. As we explained in Paragraph 
542 of Part 1 the selectivity of a receiver using tuned radio fre¬ 
quency amplification is very high and is in proportion to its 
audibility. 

Easy to Operate: The operation of the Harkness Receiver 
is exceedingly simple. The circuit has only two variable controls; 
the set has just two dials to turn. Each control is independent 
of the other and the dial settings for different stations are per¬ 
manently accurate. The fact that the receiver does not oscillate 
also greatly simplifies the operation. 


































160 


CONSTRUCTION OF RECEIVERS 


Cost Remarkably Low: The cost of the Harkness Receiver 
is extremely low as compared with other receiving systems. The 
circuit and design are such that a very small amount of apparatus 
is required to gain high audibility and selectivity. In fact, the 
efficiency of the single-tube Harkness Receiver, which can be 
built for only twenty-five dollars, is practically equal to that of 
a standard 3-tube set costing three times as much. 

No Whistles or Squeals—Cannot Reradiate: When built 
with the special apparatus designed for the circuit the Harkness 
Receiver does not oscillate and therefore does not generate a 
single whistle or squeal or cause interference to others by re- 
radiation. 

WHY THE HARKNESS RECEIVER DOES NOT 
OSCILLATE. 

The Harkness Receiver has no potentiometer or neutralizing 
condenser to stop self-oscillation and although the grid of the 
reflex tube is connected directly to the negative side of the fila¬ 
ment the receiver does not generate continuous oscillations. 

How is this possible? The ordinary reflex receiver requires 
a grid potentiometer to stop self-oscillation and other systems— 
notably the radio frequency amplifiers developed by the British 
and French during the war—use neutralizing condensers or other 
methods of producing negative reaction to stop self-oscillation. 
(See Par. 479, Part 1.) The Neutrodyne receiver uses the latter 
system. Why, then, does the Harkness Receiver not oscillate? 

To understand the reason for this it is necessary to appre¬ 
ciate how self-oscillation is caused. Continuous oscillations in a 
radio frequency amplifier are generated by the feed-back of 
energy from one circuit to a preceding circuit through some form 
of coupling. The important sources of coupling in a radio fre- 
quenced amplifier are (1) the inductive coupling due to magnetic 
linkages between the radio frequency transformers and (2) the 
capacitive couplings between the plate and grid of each radio fre¬ 
quency amplifying tube. The first coupling can be avoided by 
correctly designing the transformers and the arrangement of the 
transformers in the amplifier. The second coupling cannot be 
avoided without using special tubes, and then only to a limited 
degree. 

The first maxim of radio frequency amplifier design is to 
choose and arrange the apparatus so that all sources of coup¬ 
ling are reduced to an absolute minimum. (See Par. 489, Part 
1 .) But if the unavoidable feed-back produced by the capacitive 
coupling of the radio frequency amplifying tube or tubes is strong 
enough in itself to generate continuous oscillations these oscilla¬ 
tions can only be controlled by using a potentiometer or neutraliz¬ 
ing condenser. 

Ordinary reflex receivers and other systems using radio fre¬ 
quency transformers with tuned or semi-tuned primaries use a 
potentiometer or neutralizing condenser because the strong oscil¬ 
lations set up across the resonant plate circuit of the amplifying 
tube feeds back sufficient energy through the self-capacity of the 
tube to generate continuous oscillations. Even though every pre¬ 
caution is taken to avoid inductive coupling between the trans- 


THE HARKNESS RECEIVER 


161 


formers, the feed-back through the tube, which cannot be avoided, 
is strong enough in itself to cause self-oscillation. 

But the Harkness Receiver, in common with the Neutro- 
dyne a.nd others, uses radio frequency transformers with aperio¬ 
dic primaries. (See Par. 456, Part 1.) The oscillations set up 
across the primary of the radio frequency transformer T2 in the 
Harkness Receiver do not feed back sufficient energy through the 
tube to generate continuous oscillations. Of course, if the 
receiver were poorly designed and close inductive coupling ex¬ 
isted between the transformers T1 and T2, the combined feed¬ 
back might be sufficient to cause self-oscillation, but this, of 
course, is not the case; the receiver is designed so that the induc¬ 
tive coupling between the transformers T1 and T2 is very small. 

The Harkness Receiver, then, does not use any of the cus¬ 
tomary methods of preventing self-oscillating because it is not 
necessary to do so—provided the transformers T1 and T2 are 
correctly designed and provided also that the apparatus is ar¬ 
ranged so that all avoidable coupling is reduced to a minimum. 

As proof of this explanation we refer the reader to the previ¬ 
ous lesson in which we demonstrated that the neutralizing con¬ 
densers of the “Neutrodyne” are unnecessary. The “Neutro- 
dyne” oscillates because inductive coupling exists between the 
radio frequency transformers. The feed-back of energy through 
the tubes is not sufficient in itself to generate continuous oscilla¬ 
tions. 

SUCCESSFUL OPERATION DEPENDS ON SPECIAL 
PARTS 

It will be evident from the foregoing that the successful 
operation of the Harkness Receiver largely depends upon the 
design of the radio frequency transformers T1 and T2. The 
number of turns on the primary of T2 must be such that good 
amplification is obtained without causing self-oscillation. More¬ 
over, the number of turns on the secondary of T2, the constants 
of T2, the capacity, resistance and losses of the variable con¬ 
densers all affect the operation of the receiver. In previous publi¬ 
cations and articles on this receiver we have given exact details 
of how the transformers T1 and T2 are made but we found that 
this led to trouble. Some amateur constructors wound coils ac¬ 
cording to our specifications but failed to achieve success with 
their receivers because the capacity or resistance of the con¬ 
densers they used changed the constants of the circuit. We 
found it difficult to convince these amateurs that the fault was 
theirs and not ours! 

To ensure the successful operation of home built sets, the 
Author has designed complete radio frequency transformer units 
for use with the Harkness Circuit. These units are known as 
“Harkness Flexoformers” and, being simple, they are inexpen¬ 
sive. The Flexoformer, as shown in Fig. 158, consists of an air 
core radio frequency transformer attached to the rear of a special 
variable condenser with extremely low electrical losses and low 
minimum capacity. The units are made in two types. Flexo¬ 
former T1 is used to couple the antenna circuit to the grid cir¬ 
cuit while Flexoformer T2 is used to couple the plate circuit to 


162 


CONSTRUCTION OF RECEIVERS 



Fig. 158. The Harkness 
Flexoformer — Heart of the 
Harkness Receiver. Two of 
these special units are neces- 
to build the receiver. 
Note the design of the Con¬ 
denser. The stator and rotor 
are die-cast. The air core 
transformer is designed for use 
with this condenser only. To¬ 
gether they make up the com¬ 
plete Flexoformer. 


Fig. 159. The 

Harkness Audio Fre¬ 
quency Transformer is 
especially designed for 
the Harkness Receiver. 
This transformer is 
largely responsible for 
the high tone quality 
of the Harkness Re¬ 
ceiver and its freedom 
from distortion. 









































THE HARKNESS RECEIVER 


163 


the rectifying circuit. (See Fig. 157.) A receiver built with these 
two units and wired correctly is bound to operate successfully. 

Special Audio Transformer Aids Quality of Reproduction: 
In experimenting with the Harkness Circuit we found that the 
design of the audio frequency transformer affected both the ampli¬ 
fication of the system and the quality of the reproduction. Some 
standard makes of transformers were found to be entirely un¬ 
suitable. The Author therefore designed a special audio fre¬ 
quency transformer for use in the circuit and best results are had 
when this transformer is employed. Fig. 159 is a photograph of 
the transformer. While especially designed for the Harkness 
Circuit this transformer, of course, can also be used in any audio 
frequency amplifying circuit. 

THE TWO MODELS OF HARKNESS RECEIVER. 

Harkness Receivers are at present made in two models. 
Model A uses only one tube, consistent with the fundamental cir¬ 
cuit, while Model B uses two tubes. The second tube of Model B 
is a plain audio frequency amplifier and the effect of this addi¬ 
tional tube is to greatly increase the audibility of the receiver. 

In the remaining pages of this lesson we show how the 
reader can build either of these receivers. 

HOW TO BUILD THE HARKNESS RECEIVER 
MODEL A. 

List of Parts: Below is a complete list of the parts which 
are required to build the single-tube Harkness Receiver, Model 
A: 

1 Harkness Flexoformer T1 
1 Harkness Flexoformer T2 
1 Front Panel, 7 in. x 12 in. (See Fig. 160) 

1 Shelf Panel, 4 in. x 7^4 in. with tube socket and two 
mounting brackets. (See Fig. 162) 

1 Harkness Audio Frequency Transformer 
1 Harkness Crystal Detector. (See Fig. 161) 

1 Filko9tat 

1 Filament Control Jack (Single Circuit) 

4 Binding Posts 

2 Dials (4 in. in diameter) 

Wire and Insulating Tubing. 

The front panel is shown in Fig. 160, which also indicates 
how this panel is drilled. Fig. 162 shows the shelf panel with 
the tube socket and mounting brackets attached. Particular at¬ 
tention is drawn to the crystal detector, Fig. 161, which was 
especially designed for the Harkness Receiver. This vernier con¬ 
trol panel-mounting crystal detector gives perfect satisfaction. 
A sensitive adjustment of the catwhisker can easily be found in 
a moment by turning the lower knob. To find a new surface the 
crystal itself can be revolved by turning the upper knob. 

Assembly and Wiring. 

The photographs and drawings which appear on Pages 165, 
166 and 167 illustrate the progressive steps in the assembly and 
wiring of the single-tube Harkness Receiver. Clearer than 
words can convey these illustrations reveal the simplicity of this 
receiver and the ease with which it may be constructed. 


164 


CONSTRUCTION OF RECEIVERS 





F ip- 160 (above). The front panel of 
the Harkness Receiver measures 7" x 12" 
ami is drilled as indicated. 


Fig. 161 (left). This crystal detector 
was especially designed for use in the 
Harkness Receiver. The design is simple 
and yet original. On the front of the 
panel there are only two little knobs visible. 
The upper knob revolves the crystal itself 
while the lower knob adjusts the tension 
on the catwhisker. This crystal detector 
makes the finding of a sensitive point a 
simple matter. 


Fig. 162 (below). The rear panel of the 
1-tube Harkness Receiver is shown in this 
photograph. The panel measures 4" x 7%" 
and is used to support the tube socket, 
audio transformer and binding post ter¬ 
minals. 


\ 





THE HARKNESS /RECEIVER 


165 



Model A 


Front Panel 


Rear Panel 


Assembly Complete 






166 


CONSTRUCTION OF RECEIVERS 




:o- 


SSm. 

Mi 




Fig. 166. This shows how the Harkness Receiver appears when it is 
completely assembled, the three progressive steps in assembly having 
been clearly set forth on the preceding page. 


Fig. 167. This bottom view of the single-tube receiver shows how the 
wiring is made. Soft-drawn copper wire, covered with “spaghetti” is 
used for making the connections. ' 
























THE HARKNESS RECEIVER 


167 


Wiring the Hark ness Receiver 
Model 


THe 


One Tube Harkness Circuit 





































168 


CONSTRUCTION OF RECEIVERS 


HOW TO BUILD THE HARKNESS RECEIVER 
MODEL B. 

The 2-tube Model of Harkness Receiver has proved to be 
one of the most popular applications of the circuit. This model 
embodies the circuit of Model A with an additional stage of audio 
frequency amplification. The extra tube so greatly increases the 
audibility of the receiver that stations within a radius of 1000 to 
1500 miles are consistently received with sufficient volume to 
operate a loudspeaker. 


List of Parts: Below is a complete list of the parts required 
to build Model B : 

1 Harkness Flexoformer T1 

1 Harkness Flexoformer T2 

1 Front Panel, 7 in. x 12 in. 

1 Shelf Panel 4 in. x 7^ in. with 2 tube sockets and two 
mounting brackets. 

2 Harkness Audio Frequency Transformers 

1 Harkness Crystal Detector 

1 Filkostat 

1 Filament Control Jack (Single Circuit) 

4 Binding Posts 

2 Dials (4 in. in diameter) 

Wire and Insulating Tubing. 

The front panel of Model B is exactly the same size and is 
drilled in the same way as the front panel of Model A. The rear 
panel, however, has two vacuum tube sockets turned into its 
surface and is drilled to support two audio frequency transf^m- 
ers as well as the mounting brackets and binding posts. 


Assembly and Wiring. 

The 2-tube Harkness Receiver is just as easy to build as the 
single tube model. There are three progressive steps in the 
work of assembly, as illustrated by the photographs appearing 
on the opposite page. First, the Flexoformers, crystal detector, 
Filkostat and telephone jack are mounted on the front panel in 
the positions indicated. Second, the audio frequency transform¬ 
ers, mounting brackets and binding posts are mounted on the 
rear panel. Third, the rear panel is secured to the front panel 
by screws passing through the front panel and tightening into 
the threaded mounting brackets. 

The photographs and drawings on Page 170 and 171 show 
how to wire the completely assembled receiver. 


THE HARKNESS RECEIVER 


169 















CONSTRUCTION OF RECEIVERS 





Fio 173 This view of the completed 2-tube Harkness Receiver shows 
tlie arrangement of Us parts very clearly. Plexofonner 11 appears on 
the left and T2 on the right. 


Fig. 174. This bottom view of the finished model B shows how the 
wiring is made. All joints are carefully soldered. 





































THE HARKNESS RECEIVER 


171 




Crystal Detector^ 


\7 Hardness 

/ Flexofoiuncn Tl 

V A 


Hdrfcficss 
Flexotor merTz 


Hardness 

Audio Transformer 


^ Hd pI;ii<>m 
^ Audio 

j TVdnjfoi'incr 




H<ipl:n<vss 


0-A + 

> I 


CND 


O-B+O- 


Fiot 175 and 176 The wiring of the Harkness Receiver Model B is given in these 
two diagrams Be sure to connect the wires to the correct numbered terminals as shown 
above! 1 When wiring to the binding posts on the rear panel follow the instructions given 
on Page 167. 



























































172 


THE HARKNESS RECEIVER 



The following accessories are required to operate 
the Harkness Receiver Model A (1 tube) : 

Complete antenna equipment: 

1 C 301A or U.V. 201A vacuum tube; 

1 Filament Battery (6 volts) ; 

1 Plate Battery (90 volts) ; 

1 Pair Headphones and/or Loudspeaker. 

Exactly the same accessories are required to op¬ 
erate Model B except that an extra tube is needed. 

The filament battery may be either a small storage 
battery or four dry cells. The plate battery may be 
composed of four 22J4 volt units or two 45 volt units 
connected in series. Dry cell tubes may be used 
in place of 6 volt tubes if desired although, of course, 
the audibility is much lower with the former. With 
Type C299 or U.V.199 tubes the filament battery 
should consist of three 1J4 volt cells connected in 
series. 

The illustration below shows how to connect the 
antenna, ground and batteries to the Harkness Re¬ 
ceiver (either model). The plate battery appears on 
the left and the filament battery on the right. Note 
that the negative lead of the plate battery and the 
positive lead of the filament battery both connect to 
one binding post. Similarly the negative lead of the 
filament battery and the ground lead both connect to 
the right hand post. 


































To prepare the Harkness Receiver for steady operation the 
following adjustments must be made, in the order given: 

1. Turn the lower knob of the crystal detector to the left until 
the catwhisker is resting gently on the surface of the mineral. 

2. Plug in your loudspeaker or headphones. 

3. Turn both dials until a station is heard; then turn the right 
hand dial to the position which produces the loudest sound. Then 
turn the left hand dial and reduce the volume of sound until the 
station is just audible. 

4. Turn the lower knob of the crystal detector to the right and 
left a number of times and revolve the upper knob if necessary 
until an adjustment of the crystal detector is found which gives 
loudest signals. 

The receiver is then ready for steady operation. Thereafter 
different stations can be tuned in by merely revolving the two 
large dials. When seeking a station these two controls should 
be turned simultaneously until the signal is heard; then each dial 

should be turned separately 
until the positions are found 
which produce the most vol¬ 
ume of sound. The tuning is 
just as simple as A-B-C. 

Since the dial settings for 
any particular station^ are per¬ 
manently accurate a record 
should be kept of the best po¬ 
sitions of the dials for differ¬ 
ent stations. With this record 
to refer to, any desired station 
can be tuned in by merely 
turning the two dials to the 
positions indicated. 


Keep a record like 
this of the stations 
you hear 
















Hark ness 























































































THE HARKNESS RECEIVER 


175 


ADDITIONAL OPERATING NOTES 

Although the simplified controls of the crys¬ 
tal detector in the Harkness Receiver allow a 
sensitive adjustment to be quickly and easily 
found, it is not necessary to continually 
“play” with these controls. If proper care is 
taken the crystal detector will retain a sensi¬ 
tive position for several days before requiring 
readjustment. 

The Filkostat (lower left hand knob) is 
often useful when tuning in weak signals, al¬ 
though ordinarily this control requires no 
adjustment. When a distant station is accu¬ 
rately tuned in by the two large dials the 
strength of the signal can be increased by 
gradually turning the Filkostat knob to the 
right until the receiver is in its state of maximum sensitiveness. 

THE HARKNESS CIRCUIT WITH DIODE DETECTOR 

A crystal detector is used in the Harkness Receiver, not 
merely because a three-electrode tube is more expensive, but 
because the audibility of the receiver with a crystal detector is 
just as good as when a vacuum tube is used as rectifier. Under 
these circumstances, the cost of the vacuum tube alone (without 
other necessary parts) being three times the cost of the complete 
crystal detector, there is no question that the crystal is more 
efficient for use in this circuit. 

Still another arrangement, however, can be used and a com¬ 
promise effected if desired. Instead of using either a three- 
electrode tube or a crystal as rectifier, a two-electrode tube can 
be substituted. Fig. 179 shows a tube of this type which is 
known as the “Diode.” This rectifying tube is much less costly 
than a three-electrode tube, requires no plate battery and uses 
only a single dry cell as its source of filament current. Although 
the Diode is no more sensitive than the crystal in the Harkness 
Circuit it has the advantage of requiring no adjustment what¬ 
soever. 

For the benefit of those who would like to use this type of 
rectifier in preference to a crystal detector we show a complete 
picture diagram of the popular 2-tube Harkness Circuit with 
Diode detector on the opposite page. 





LESSON 15. 


TROUBLE-SHOOTING HINTS—CARE OF RECEIVERS— 
ERECTING AN AERIAL. 


When a receiver has been constructed and wired it should 
NOT be put into operation before it is tested. If the wiring is 
incorrect or if two wires are shorting the filaments of the 
vacuum tubes may be burned out. This can be avoided by first 
testing the receiver. 

A very convenient testing device for locating trouble in a 

receiving set may be made by __ 

connecting a small 25 watt iiovoltline 
lamp in series' with the house 
supply and providing two metal 
terminals, with insulated han¬ 
dles, which will complete the zswatt 
circuit and light the lamp when 
touched together as in Fig. 

180. If the two testing term¬ 
inals are directly shorted the 
lamp will light brightly; if re¬ 
sistance is connected between 
the two terminals the lamp will 
glow at varying degrees of brilliancy depending upon the amount 
of resistance. For instance if the high resistance of the second¬ 
ary of an audio frequency transformer is between the terminals 
only a spark will be apparent when the circuit is broken—the 
lamp will not glow. Obviously, if the testing terminals are 
connected to a supposedly complete circuit and the lamp does 
not light or no. spark is discernible there is a break in the circuit. 
The location of the break can be found by testing each portion 
of the circuit. Similarly, if the terminals are applied to a com¬ 
plete circuit of high resistance and the lamp lights brightly it 
is evident there is a short in the circuit. 

When using the trouble-shooting lamp to test the circuits 
of a radio receiver do not leave the vacuum tubes in the sockets; 
remove all ground connections and make tests quickly, applying 
the test terminals for the briefest possible time. 

Testing a Receiver: After a radio receiver has been com¬ 
pletely wired, FIRST test the filament circuit by connecting 
the filament battery to the proper terminals on the receiver and 
inserting a single vacuum tube in one of the sockets. When a 



TEST 

TERMINALS 


Fig. 180 






CARE OF RECEIVERS 


177 


telephone plug is inserted in the last filament control jack and 
either the Amperite inserted or the rheostat revolved (as the 
case may be) the tube should light. If it has a rheostat this 
should vary the filament current of the tube. Repeat this proc¬ 
ess inserting the tube in each of the sockets. Then insert all 
the tubes in their respective sockets and make certain that when 
the telephone plug is removed from the last jack the filaments 
of all the tubes are extinguished. If the receiver has more than 
one filament control jack the telephone plug should be inserted 
in each jack to see if they operate properly. For instance, when 
testing the R.G. 510 receiver the insertion of the plug in the first 
jack should light the filaments of the first four tubes only; in 
the second jack the filaments of five tubes should light and in 
the last jack the filaments of all six tubes should light. 

If the filaments do not light properly it is because— 

1. The filament battery is connected to the wrong posts. 

2. The circuit wiring is imperfect or not complete. 

3. The tube socket springs are not making proper contact 
with the tubes. 

4. The spring contacts of the filament control jacks are not 
bent correctly. 

5. The rheostats or ballast resistances have a broken circuit. 

6. Some joints which appear to be connected are not making 
electrical contact. 

7. One or more of the tube filaments are burnt out. 

The remedy in most cases is obvious and can be briefly outlined 
as follows: 

1. Connect the filament battery to the correct terminals. 

2. Trace the filament wiring with frequent reference to the 
wiring diagram. Correct any mistakes. 

3. Bend up and clean off all socket springs. 

4. Inspect each automatic filament control jack and make 
certain that each functions correctly. The three springs (two 
on the last jack) furthest from the jack frame are the ones to 
which the filament wiring is connected. When the telephone 
plug is out the center spring should be making contact with the 
spring adjacent to it nearest the frame. Test with a lamp to see 
that it makes contact and, if necessary, bend the spring until it 
makes proper contact. When the plug is in, the center spring 
should be making contact with the spring furthest from the 
frame only. On the last jack the two springs furthest from the 
frame should make contact only when the plug is inserted in 
this jack Inspect the insulation between the springs of the 
jacks and clean off any solder flux with alcohol. If the insula¬ 
tion is not clean it acts as a low leakage path to both A and B 
battery currents. 

5 Test each rheostat and ballast resistance with a lamp 
touching the test terminals to the wires leading to these parts, 
not merely to the mountings. If an open circuit is found test 


178 


CARE OF RECEIVERS 


directly on the rheostat or Amperite terminals. If the circuit 
is then complete the trouble is most likely due to the connec¬ 
tions. The various connections should be tested and re-soldered 
if necessary. In the rare event that it is impossible to make a 
complete circuit through either the adjustable or automatic re¬ 
sistances replace these parts. 

6. Test across the joints with the trouble-testing lamp and 
if the lamp does not light brightly or if an arc is formed at the 
break, resolder the joints, taking care to clean the parts thor¬ 
oughly. This trouble is rare if proper care is taken when wir¬ 
ing the receiver. When it does occur it is usually found that the 
connections to the socket springs are not making proper con¬ 
tact. This can be avoided by soldering the wires to the socket 
springs so that solder flows over on the spring itself and not 
merely on the fastening screw or nut. 

When the filament circuit functions properly, connect the 
negative terminal of the plate battery to its binding post. Then, 
with all the vacuum tubes removed from their sockets, touch the 
plate battery “detector” tap to its binding post; if no sparks 
take place connect it permanently; do the same with the posi¬ 
tive plate battery connection. Then insert the telephone plug 
in any of the jacks and note whether the wire on the potenti¬ 
ometer warms up. If it does there is an error somewhere which 
allows the positive of the plate battery to be connected to the 
filament through the potentiometer. The mistake will be found 
in the plate circuits so inspect and check all plate circuit wiring. 
Make certain the plate connections are not in accidental contact 
with other leads. Then test the primaries of all radio and audio 
frequency transformers and, with all batteries removed, test for 
shorts between the primary and secondary windings. Also test 
the secondary circuits of all transformers. Similarly test the 
loop jack and its connections and the tuner circuits. 

When all circuits are tested and found correct the receiver 
may be put into operation. If, with all the exterior connections 
properly made, the receiver does not then operate, the trouble 
may be traced to a short in the telephone plug, in the phone 
leads or in the telephones themselves. 

CARE OF RECEIVERS. 

If a radio receiver is properly built and tested before being 
put into operation it requires little attention. The cover of the 
cabinet should be kept closed to prevent dust from entering. 
Every few months the vacuum tube spring contacts should be 
cleaned off; the R.F. transformer and Amperite mountings in¬ 
spected and all binding posts tightened up. 

If a receiver which has been in successful operation sud¬ 
denly ceases to function properly the fault may usually be 
traced to 

1. RUN DOWN FILAMENT OR PLATE BATTERIES. 

2. Reversed connections to the storage battery. 

3. A burned out or “dead” vacuum tube. 

4. Loose connections to the binding posts or batteries. 

A radio receiver with a run down plate or filament battery 


CARE OF RECEIVERS 


179 


is like an automobile without gas—it won’t operate. Look there 
for the trouble FIRST. Don’t start pulling the wires apart until 
you know the batteries are in good condition and that this is not 
the reason why the set will not operate. 

Try reversing the filament battery leads. An audio fre¬ 
quency amplifier will not amplify with the filament battery leads 
connected the wrong way around. 

Some vacuum tubes last two months—others last two years. 
With normal usage the vacuum tubes of a radio receiver only 
require renewal about once a year. An individual tube, how¬ 
ever, may burn out in six or seven months and it should then be 
replaced by a new tube. A “dead” tube sometimes lights but 
operates very poorly. 

Batteries: The only way of testing a plate battery is by 
connecting a volt meter across the battery. If a 45 volt battery 
is run down to below 35 volts throw it out or at least don’t use it 
as the plate battery of a receiver. A run-down plate battery is 
responsible for most of the “static” in a good radio receiver. A 
good plate battery will last six months or more when used with 
a one, two or three tube receiver. When used with a four, five 
or six tube receiver it will rarely last more than four months— 
depending upon the average hourly use. 

A filament storage battery requires constant re-charging. 
A run-down filament battery will light the tubes of a receiver 
but this does not necessarily indicate that the receiver will func¬ 
tion properly. The voltage of the battery may be too low. 

Six volt storage batteries can be obtained in different sizes. 
A 100-120 ampere-hour battery will last longer before it requires 
re-charging than a 60-80 ampere-hour battery. The length of 
time during which current may be drawn from a fully charged 
battery depends upon the rate of discharge and the ampere-hour 
capacity of the battery. In general the rate of discharge di¬ 
vided into the ampere-hour capacity equals the number of hours 
such a current may be drawn. For instance, a five Ampere 
current can be taken from a 100 ampere-hour battery for a total 
length of time of 20 hours. If the battery is used for one hour 
a day it will last 20 days. The rate of discharge is determined by 
the type and number of tubes used in the radio receiver. Six % 
Ampere tubes (U.V. 201A-W.D.11) draw 1.5 Amperes so a fully 
charged 60 ampere-hour battery will last 40 hours before an¬ 
other charge is required. A single 1 Ampere valve (U.V. 200- 
201) draws one ampere so a 60 ampere-hour battery should last 
60 hours before another charge is necessary. 

However, a battery should never be completely discharged. 
It should be charged frequently. Battery chargers can be pur¬ 
chased for this purpose and the house electric light supply used 
to charge the battery. Chargers are made for either alternating 
or direct current; the proper type must be used. 

The condition of a battery may be determined by hydrom¬ 
eter readings of the electrolyte according to the following table. 
A suitable hydrometer can easily be purchased at a small cost. 


180 


ERECTING AN AERiAL 


Hydrometer reading in 
“specific gravity’’ 
1.100 to 1.150 


Battery condition 

Discharged—charge 


immediately. 


1.200 to 1.250 


One-quarter to one-half 


1.280 to 1.300 


charged. 
Fully charged 


The condition of a storage battery can also be judged by 
testing the voltage of each cell. During the test the battery 
should be under normal load; that is to say, the current from the 
battery should be lighting the filaments of the tubes in the 
receiver. If a reading of 2.1 volts (per cell) is secured the bat¬ 
tery is fully charged. A reading of 1.8 volts indicates, that a 
recharge is necessary. 

To preserve the life of a battery the following precautions 
should be observed: 

1. The battery should never be completely discharged. 

2. The electrolyte in each cell should be kept above the top 
of the plates by regular addition of distilled water (not 
acid) to compensate for that lost through evaporation. 

3. The battery terminals should never be shorted nor should 
the battery be used with a greater load than stipulated 
by the manufacturer. 

4. The charging rate should not exceed that specified by the 
manufacturer. The charging rate of a battery divided into 
the ampere hour capacity equals the time in hours such 
a charge must be continued. To this a twenty or thirty 
percent overcharge should be added. 

5. The vent cups should be removed while charging and 
replaced immediately thereafter. 

6. The battery top should be kept clean to avoid leakage 
losses and the terminals should be cleaned regularly. 

7. Never jolt or jar the battery as the paste in the plates is 
apt to fall out and cause disastrous internal shorts. 


ERECTING AN AERIAL. 


If at all within the bounds of possibility an outside aerial 
should be used with a radio receiver. Granted the receiver may 
be so sensitive that signals can be picked up using a bed-spring, 
electric light line (with suitable attachment) a loop or other in¬ 
door antenna but if an outside aerial can easily be erected the 
receiver is not being operated with maximum efficiency and an 
unnecessary amount of amplification is being used for local 
stations. 

The erection of an aerial is usually a simple matter. The 
following equipment is required: 

100 feet #14 bare copper wire 
50 feet #18 insulated lead-in wire. 

3 Insulators 

1 Porcelain lead-in insulator 
1 Ground clamp 
1 Lightning Arrester 


ERECTING AN AERIAL 


181 



Figs. 181-184 

The drawings of Figs. 181 to 186 suggest different ways of 
erecting the aerial, according to the location. The wire, of 
course should not touch the buildings but should be held off 


































182 


ERECTING AN AERIAL 


by means of the insulators. The insulated lead-in wire should 
be brought into the house through the porcelain insulator and 
connected to the ‘'Antenna” post of the receiver. The “Ground” 
post of the receiver should be connected to a water-pipe or other 
connection which goes to ground by means of the ground-clamp. 




The water-pipe or radiator is usually the most convenient ground; 
the pipe should be cleaned thoroughly before attaching the 
ground clamp. The wire from the “Ground” post of the re¬ 
ceiver should be soldered to the ground clamp. A good ground 
is just as important as a good aerial. 

The lightning arrester is connected between the antenna 
and ground. This must be inserted to conform with fire regula¬ 
tions and to protect the receiver in case lightning strikes the 
aerial. Incidentally, the aerial then acts as a lightning protec¬ 
tor. If lightning does strike the aerial the house will not be 
damaged whereas without the aerial the lightning might strike 
the house itself. 












































7he Am&jmg X(W 

Harkness Coupler 

maker selective reception easy 


The Harkness Coupler was specially conceived, specially designed and 
specially built to cut out interference. That’s what it was made for and 
that’s what it does! 


There is nothing uncanny or mysterious about it. The explanation is 
simple. Vario-couplers were originally designed for non-regeneratiVe 
receivers. When regenerative and radio frequency amplifying sets came 
into use the design remained practically the same. But Kenneth Harkness 
realized that the coupling variation of these old-fashioned couplers was alto¬ 
gether too close for the modern type of receiver. With his staff of experts 
he set to work to devise a new coupler which would meet the new condi¬ 
tions. The Harkness Coupler was the result. The coupling variation of this 
instrument is scientifically correct for regenerative and radio frequency ampli¬ 
fying receivers. The use of this coupler enormously increases the selectivity 
and efficiency of a receiving set. It will banish interference from your 
receiver the minute you hook it up in your circuit. It will impart to your 
set a razor-sharp selectivity which you never dreamed possible. With just 
a hair’s breadth turn of your dial you will be able to completely tune out an 
interfering station and bring in the one you want—without a trace of inter¬ 
ference. 

SPECIFICATIONS 

To mount on the front panel of a receiver the Harkness Coupler requires a clearance space of iVi 
inches in height, 4 inches in width and 7 inches in depth. The primary coil is wound at one end of a 
piece of Formica tubing 6 inches long and 4 inches in diameter. The primary coil has 5 taps. The 
secondary is wound on a smaller Formica tube 2% inches long and 3 inches in diameter. Green double 
silk covered wire is used for both coils. Wave-length range of the coupler with a .0003 mfd. condenser 
across the secondary is 180 to 575 meters. The rotor is arranged so that a complete half turn of the 
coupling dial is required to vary the coupling from zero to maximum. Pig-tail connections are used 
from the rotor to avoid electrical losses. The heavy, durable hardware is beautifully nickel-plated. Ship¬ 
ping weight about 2 lbs. 


STOCK NO. 300—Harkness Coupler, Type A. For use with receiver*,— —- 
having two or more stages of untuned radio frequency amplification. «p5.75 

STOCK NO. 306—Harkness Coupler, Type C. For use with regener¬ 
ative sets or receivers with one stage of untuned radio frequency ampli¬ 
fication . 


Satisfaction Guaranteed 










1 



llllllllllllllllllllllllllllllll 

Full instructions tell¬ 
ing how to connect and 
use the Harkness Coup¬ 
ler are enclosed with 
each coupler; also cir¬ 
cuit diagrams of radio 
and audio frequency 
amplifiers. 

IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII 



Don’t endure interference any longer! Install a Harkness Coupler 
in your set or build yourself a “Harkness Tuner” and tune out interfer¬ 
ence! We don’t claim the impossible for this remarkable coupler, but we 
do guarantee that it will make a radio frequency amplifying receiver at 
least 100% more selective than any other tuning device and that it will 
considerably increase the selectivity and facilitate the operation of a 
regenerative receiver. 

Avoid imitations of the Harkness Coupler. In common with other 
new and original developments of the Radio Guild the Harkness Coupler 
is being imitated by the parasite “manufacturers” of the radio business. 
Ask your dealer for the Radio Guild Harkness Coupler and look for the 
Guild Seal on the label. 


If Your Dealer Cannot Supply You Promptly 


llse Order Form On Last PaR*c 1 













When you have DX transformers in your receiver 
you’ll be able to pick up distant stations just as easily 
as you pick up the local ones now! These trans¬ 
formers are so perfectly designed that they bring in 
distant stations just as if they were next door to you. 


IT PAYS TO USE THE BEST—AND THE DX TRANSFORMER 

IS THE BEST 

After all, the most important part of your radio frequency amplifying 
receiver is the transformer. If you use an inferior, imitation transformer 
you can’t expect to get good results. 

Yet, do you realize that nearly every radio frequency transformer on 
the market today is a poor imitation of the first and original radio fre¬ 
quency transformer—the DX? 


SUPERIOR DESIGN COVERED BY BASIC PATENTS 


The DX transformer was designed by Mr. P. D. Lowell of the 
United States Bureau of Standards, Washington, D. C. There are basic 
patents on the design of the transformer, and it is this patented design— 
which cannot be imitated—which makes the DX transformer superior to 
all others. 


OUR GUARANTEE PROTECTS YOU FROM ALL RISK 

' : - • • 

As manufacturer’s distributors we unconditionally guarantee every 
DX transformer we sell. We know that these transformers will more 
than satisfy you. In fact, if they don’t meet up with tyour expectations in 
every way; if you don’t think that the DX transformers you buy from us 
are infinitely superior to any radio frequency transformers on the market, 
we will take them back and refund every cent of your money. We can’t 
do more than this to demonstrate our confidence in this really wonderful 
product. When you order DX transformers from us you take absolutely 
no risk; we guarantee them to the hilt—and our guarantee is without a 
loophole. 

Choose now the types of DX transformers you need for your receiver 
from the list on the next page—then get your order in without delay. 



Minuhccarers Dealers =write 

adioGuild' . , , , n 

Distributors for Discounts 













Plugin Motuihiur Feature 


The DX transformer is not only electrically perfect— it is mechanically perfect, 
too. If you want to change the wave-length range of your receiver, you don’t 
have to tear it apart to do so. With the DX transformer’s exclusive, patented 
plug-in mounting feature all you have to do is to detach the transformer of one 
wave length range and substitute another. No fuss—no bother; just like a plug 
and jack and just as speedy and simple. 


Full instructions and circuit diagrams 
are enclosed with each instrument. All 
types have the same dimensions—Length. 
4 inches; Width, 1% inches; Depth. lVi 
inches. Weight, 8 ozs. 


TYPE DX-12 

Wave length range 220 to 550 meters. 
For the reception of all broadcasting. 

Stock No. 312—DX-12 Transformer, 
with 4 mounting lugs.$6.40 

TYPE DX-S 

Wave-length range 400 to 1,200 meters 
Particularly useful for ship and commer¬ 
cial radio operators. 

Stock No. 318—-DX-S Transformer, with 
4 mounting lugs..$6.60 

TYPE DX-2 

Wave-length range 900 to 3,000 meters. 
Receives Arlington Time Signals, etc. 

Stock No. 320—DX-2 Transformers, 
with 4 mounting lugs.$6.60 

TYPE DX-2H FOR SUPER 
HETERODYNE 

Especially designed for the Super- 
Heterodyne Receiver, this type of DX 
transformer, with a peak wave-length of 
5,000 meters, is widely recognized as the 
most efficient transformer yet produced. 

Stock No. 322—DX-2H Transformer, 
with 4 mounting lugs.$6.60 

TYPE DX-INPUT 

Designed for the input of a Super- 
Heterodyne amplifier. Similar to the 
DX-2H except that it is tuned to the 
peak wave-length of the DX-2H, thereby 
insuring high selectivity. 

Stock No. 324—DX-INPUT Transform¬ 
er, with 4 mounting lugs.$6.60 

Stock No. 326—DX Mounting.90c 


If Your Dealer Cannot Supply You Promptly 


Use Ord er F orm On Last Pag*e 
















\V/1 


mmumum 




\ 




n 


The Supreme Creation of Radio’s Foremost Designer 
of Receiving Apparatus 


/V 


If this receiver were on your desk right in front of you, you 
would have at your command the power to receive every broadcasting 
station in the United States. 

Read that again. Realize what it means to you. Imagine the 
fascination of plugging your phones or loud speaker in one of those 
jacks on the right hand side of the panel and picking up, one after 
another, various broadcasting stations in different parts of the country. 
It doesn’t matter where you live. The R. G. 510 can reach out from 
the East to the West, from the North to the South, from wherever you 
live to the furthermost ends of the continent. With its seemingly 
limitless power of amplification the R. G. 510 can pick up the feeblest 
signal and reproduce it for you with the volume of a phonograph. 

Still more important, the selectivity of the R. G. 510 is in propor¬ 
tion to its audibility. This set is positively tne first multi-stage radio 
frequency amplifying receiver with which an outside aerial can be used 
to the fullest advantage without experiencing interference. The R. G. 
510 is just as selective with an outside aerial as it is with a loop. 

When you consider the unusual merits of the R. G. 510, the high quality of the 
parts used in its construction, the superfine workmanship of the Guild craftsmen who 
make these parts and who assemble and wire the receiver, you will agree that the 
price at which you can buy this set is amazingly low—only $135. This special low 
price is made to bring this product of the Radio Guild to the public attention. We 
can only supply a limited number of receivers at this price. If you are to be one of 
those who will take advantage of this special introductory price you must order your 
R. G. 510 today—'NOW. 

SPECIFICATIONS 

Tuner: Harkness Coupler with two Radio Guild leak-proof condensers and inductance switch 
set. Binding posts provided for inserting loading coils. Loop jack on amplifying unit. 

Amplifier: Ampli-Unit with 2 stage DX transformer coupled radio amplifier, tube detector 
circuit and 3 stage Radio Guild transformer coupled audio amplifier; wire potentiometer; Filko- 
stat filament control of R. F. and detector circuit; Amperite automatic control of audio amplifier 
filaments; Fil. control jacks on 1st, 2nd and 3rd audio stages; battery binding posts along rear. 

General: Formica panels, fully engraved: polished black composition 3%-inch dials; mahog¬ 
any cabinet with highly polished finish. Length, 26 inches; Height, 10% inches; Depth, 8% 
inches. Shipping Weight about 20 lbs. 

Stock No. 510 -R. G. 510 Receiver. $135.00 


If Your Dealer Cannot Supply You Promptly 


ms. Order Form On Last Pagre 11 





















You Add the Finishing Touches—and Save Money 


You will admit that the price of the complete R. G. 510 receiver is extremely low for 
a high quality 6-tube set of this type. Yet we realize that many readers of this book may 
not be able to afford the expense. We want to do everything we can to bring the price 
of this set within your means—so we are making you a special money-saving offer. We 
will supply you with the R. G. 510 in 3 , partly finished state and let you complete the 
work of construction yourself. In this way we can reduce our manufacturing cost and 
you can purchase the partly finished R. G. 510 for $15 less than the price of the com¬ 
pleted model. 


This receiver is manufactured in two separate operations. In the first operation the 
amplifying unit is assembled on the front panel and the amplifier completely wired and 
tested. In the second operation the tuner is assembled and wired and the complete set 
tested Now if you care to perform the second operation yourself you can save the 
labor cost of’ $15.00. All you have to do is to assemble the tuner parts (supplied with 
each partly-finished outfit) and make a few simple connections. Full instructions explain¬ 
ing how to finish the work of construction are given in Lesson 10 of this book. The 
whole job will only take you about 30 minutes and yet you will save $15 and get this 
wonderful receiver at a bargain price. 


This money-saving offer to supply you with a partly finished R. G. 510 receiver is 
not advertised in any other publication—it is made to readers; of this book only. NOW 
is your opportunity to buy the R. G. 510 at a price within your reach but you must 
act quickly as we cannot hold this special offer open indefinitely. Get your order m 

today ‘ SPECIFICATIONS 


Finished Unit: The standard 6-tube $10 Ampli-Unit is supplied, mounted on the completely 

drilled finished and engraved front panel. The Ampli-Unit i* wired and tested, ready for you to 

use individual Parts to be assembled and wired by you: With the finished amplifier and front 

nanel are supplied all the remaining parts required to finish the construction of the receiver These 

panel are supimeu “ . Pounler 2 Radio Guild leak-proof condensers, inductance switch set, 2 
binding^ "Sts^%? e feS ?f w“e C S 2’len5h 9 insulating tubing, 3 di.lt and the mahogany cabinet to 
enclose the finished set. Shipping Weight about 30 lbs. 


STOCK NO. 512 —Partly finished R. G. 510 Receiver. 


$120.00 


If Your Dealer Cannot Supply You Promptly 


































miosGreatestAchievement «« 



Operates a Loud Speaker with One Tube—Breaks All 

Receiving Records! 

2,000 miles is one of the actual distance records of the amazing new 
Harkness Receiver! 2,000 miles! You will notice we don’t say “thousands 
of miles”; we don’t say “across the continent” or make any other vague, 
uncertain claim which cannot be proven. This is a specific example of 
what the Harkness Receiver has done and here is the proof. Mr. Stephen 
E. Merrill of Port Richmond, N. Y., writes: 

“Your set sure is a wonder! Station CFCN, Calgary, Alberta, 
over 2,000 miles away, comes in regularly. Sometimes this station is 
so loud that I can use my loudspeaker.” 

Here is another example of long distance record-breaking reception. 
Mr. Thos. Johnson, of Wilmington, Del., writes: 

“I have received nearly every broadcasting station in the United 
States and have also heard 2LO —London, England. If anyone wants 
to know what the Harkness Receiver will do—tell them to write me.” 

Our claim that the Harkness Receiver operates like other sets costing 
two and three times as much is corroborated by scores and scores of 
Harkness Receiver owners who KNOW this to be a fact. For instance, 
Mr. D. J. Gilbert, of Port Byron, N. Y., writes: 

“There are some 12 or 15 outfits in this locality costing from 
$150 to $225 each—yet my little one-tube Harkness Receiver can 
reach out and get stations which the others cannot touch.” 

/ * 

Easy to Operate—No Whistles or Squeals 

Most owners of Harkness Receivers are particularly impressed by the 
ease and simplicity with which they can receive distant stations. Below 
are quotations from a letter typical of hundreds of similar letters on our 
files: 

“ . . . I could hardly believe my earsl Louder than a 2-tube 
set, without a single squeal or whistle and with a simplicity in tuning 
which is almost uncanny, I believe the Harkness Receiver to be the 
greatest one in its class.”—H. Mohr, Cleveland, Ohio. 

Go to your dealer today and ask him to show you either a one or 
two-tube model of Radio Guild Harkness Receiver. A demonstration will 
convince you. If your dealer cannot supply you, use our order form and 
we will ship you a Harkness Receiver with our guarantee that if it does 
not meet up with your expectations in every way we will refund every 
cent of your money. 

SINGLE-TUBE HARKNESS RECEIVER, MODEL A 
All parts used in the construction of this set are of the finest quality, 
most of them being made in our own factory. The cabinet is polished 
oak. Length 13 inches, Height 10 inches, Depth 7 inches. Panel 7 
irnies by 12 inches. Shipping Weight 10 pounds. 

STOCK NO. 556—Harkness Receiver, Model A. 

TWO-TUBE HARKNESS RECEIVER, MODEL B 

The external appearance and dimensions same as Model A. Ship¬ 
ping weight 11 pounds. 

STOCK NO. 566—Harkness Receiver, Model B. 


$4250 

$52so 


If Your Dealer Cannot Supply You Promptly 


Use Order Form On Last Page _ j 







































(S©iri£&im 


The Harkness Receiver 
you build with the Radio 
Guild box of standardized 
parts is bound to operate 
successfully because with 
these parts you can build 
an exact duplicate of the 
remarkable receiver which 
Kenneth Harkness de¬ 
signed. Guess work is 
eliminated. The panels are 
drilled and ready for you 
to assemble. Each part 
is standardized and can 
only go in its proper 
place. Moreover each 
part is the correct type to 
use—the type which Ken¬ 
neth Harkness designed as 
the result of months of 
experimentation. 


Construction Easy 


The Radio Guild box of standardized parts 
makes it easy for you to build the Harkness 
Receiver. Each box contains ALL the parts 
to build the set, and each part is specially 
prepared and standardized to simplify the 
work of construction. The panels are 
drilled; the coils are wound; the terminals 
are numbered; each box of parts contains 
every necessary item right down to the last 
screw. With only a screwdriver you can 
put the whole set together in just a few 
minutes. 


j®s<r Baur^aimHi MWa® I 


When you buy a box of genuine Radio Guild Hark¬ 
ness parts you get big value for your money. Each 
part is manufactured in enormous quantities under 
the most modern system of standardized production. 
It stands to reason that we can sell these parts for a 
very low price and still maintain the Radio Guild 
standard of quality. 

But apart from the low price of this box of parts itself—consider also the value of 
the receiver which you can build with these parts. Remember—if you were to buy a 
receiver of any other type with the receiving range, volume and selectivity of the 
Harkness Receiver you would have to pay at least three or four times as much! This 
box of standardized parts is actually the greatest bargain in radio today. You get 
more real value for your money than any other investment in radio material you 
could possibly make. 

For a limited time we are featuring the prices of the Boxes of Harkness Parts. 
How long these extremely low prices will last we cannot say. We may have to raise 
them any day. Tomorrow may be too late to buy at these bargain figures—so to take 
advantage of our special rock-bottom prices, buy your box of parts today—NOW! 




SSXNX\\\\\\\X\\\\XX\\^^ 



Bffisr <5>f JPauts Jf®ff ©m® Tmlb® Set' 

This attractive box contains all the parts to build the one tube Harkness 
Receiver, Model A, as follows: 1 Flexoformer Tl; 1 Flexoformer T2; 1 Front 
Panel, completely drilled; 1 Shelf Panel with tube socket and two mounting 
brackets, panel completely drilled; 1 Harkness Audio Transformer; 1 Harkness 
Crystal Detector; 1 Filkostat; 1 Single Circuit Fil. Control Jack; 4 Binding Posts; 

2 Dials (4 in.); 25 ft. copper wire; 2 lengths insulating tubing; Instruction 
Booklet. Shipping Weight. 8 lbs. 

STOCK NO. 558 —Complete Harkness Parts, Model A. 

Gmifhtos to ®IF Part 

Every necessary part to build the 2-tube Harkness Receiver, Model B. is 
contained in this box, as follows: 1 Flexoformer Tl; 1 Flexoformer T2; 1 Front 
Panel, completely drilled; 1 Shelf Panel with 2 tube sockets and mounting 
brackets, panel completely drilled; 2 Harkness Audio Transformers; 1 Harkness 
Crystal Detector; 1 Filkostat; 1 Single Cir. Fil. Control .Jack; 4 Binding Posts; 

2 Dials (4 in.); 25 ft. Copper Wire; 3 lengths Insulating Tubing; Instruction 
Booklet. Shipping Weight, 8 lbs. 

I STOCK NO. 568 —Complete Harkness Parts, Model B. . 


If Your Dealer Cannot Supply You Promptly 


I Use Order Form On Last Page 







































iiiiiiiiiMimiiiiiiiiiii 

Avoid worthless imita¬ 
tions of this product. Ask 

for the Radio Guild Hark- 
ness Flexoformers and look 
for the Guild Seal on the 
package. 

Illlllllllllllllllllllllll 


Cast Metal 
Condenser Plates 
Cannot Short Circuit 


To build the Harkness Receiver a pair of 
Flexoformers (Types T1 and T2) are absolutely 
essential. The Flexoformer consists of a special 
air-core transformer mounted on the rear of the 
Radio Guild low-resistance leak-proof condenser. 
The whole secret of the amazing efficiency of the 
Harkness Receiver lies in the design of this special 
unit. 

SPECIFICATIONS 

Transformer: —Wound with green D. S. C. wire 
on Formica tubing 2% inches diam., 2 inches 
long. Each terminal has soldering lug. All ter¬ 
minals numbered making correct wiring of re¬ 
ceiver easy; numbered' circuit diagrame accom¬ 
pany each Flexoformer. Condenser: The most 
perfect instrument ever produced. Resistance 
and other losses lower than any other. Wave 
length range complete Flexoformer 220 to 575 
meters. Shipping weight 1% lbs. each. Ship¬ 
ping weight of coils 12 ozs. per pair. 

STOCK NO. 100 —Harkness Coils, without con- <1*0 on 
densers, per pair.«p«).UU 

STOCK NO. 102 —Harkness Flexoformer Tl, 
complete with Radio Guild leak-proof condenser,QQ 

STOCK NO. 104 —Harkness Flexoformer T2, 
complete with Radio Guild leak-proof condenser ,qq 


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Two All Essential Items 


^Vew 






fifllMHlaSl r Vernier Control 

v Crystal Detector 






Turn this knob 
dnd you have 
the desired 
contact 


4 ^Vetv 


So-called permanent crystal detectors are 
not sensitive enough for the Harkness Re¬ 
ceiver and adjustable detectors of the old- 
fashioned type are too troublesome to op¬ 
erate. So we designed this new panel¬ 
mounting Crystal Detector which is highly 
sensitive, easy to adjust and almost perma¬ 
nent In action. To adjust the new Radio 
Guild Crystal Detector you just turn two 
little knobs; one knob revolves the crystal 
itself and the other delicately adjusts the 
tension on the “catwhisker." A sensitive 
point can be found with one little twist of 
the lower knob—and the adjustment stays 
put for days at a time. 

SPECIFICATIONS 

This Detector is supplied completely assem¬ 
bled on a drilling template, as Illustrated. 
You can mount it on your panel in just a 
few moments. All metal parts are nickel- 
plated. Total height, 2% inches. Depth 
clearance behind panel, 1% inches. Ship¬ 
ping Weight, 1 lb. 

STOCK NO. 116— Crystal De¬ 
tector (without mineral). 


$ 2.00 


Especially Designed 
Harkness Receiver 


This transformer is largely responsible for 
the high amplification and pure quality of 
reproduction for which the Harkness Receiver 
is famous. Other transformers may or may 
not function in the Harkness Circuit. If 
you are building a Harkness set—take no 
chances; use the transformer which Mr. 
Harkness designed especially for this circuit. 
The transformer, of course, can also be used 
in standard amplifying circuits. Its uniform 
amplification of all voice and musical fre¬ 
quencies makes it the ideal transformer for 
the amplifier of. a radio broadcast receiver 
in which loud, clear, pure and undistorted 
reproduction is of the greatest importance. 
The coil windings have a ratio of 4% to 1. 
Circuit diagrams showing how to use the 
transformer for all purposes are enclosed with 
each instrument. Height, 2 3/16 Inches. 
Length, 2% inches. Shipping weight, 1% lbs. 

STOCK NO. 120—Audio 
Transformer. 


$4.85 <"• 


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Form On Last Pagre 


se 













Build Your Own Tuned Radio Frequency Receiver 

With three of these now Double-Dee transformer units and a few accessories you 
can easily build a receiver with two stages of tuned radio frequency amplification. 
Kenneth Harkness, designer of the Double-Dee transformer, tells you how to build a 
4-tube set of this type in Lesson 12 of this book. The Double-Dee transformer solves 
the problem of preventing self-oscillation in a 2-stage radio frequency amplifying 
receiver. The inductive coupling between transformers is eliminated and the Double- 
Dee units are designed to give maximum amplification without producing continuoqs 
oscillations. No potentiometer or neutralizing condensers are required. Full instruc¬ 
tions and circuit diagrams, showing how to connect to the numbered terminals of the 
Double-Dee transformers, are enclosed with each instrument. 

The special air-core transformer of the “Double-Dee” is composed of two Formica 
sections shaped like the letter “D” held together by two Formica rings. The coils 
of the transformer are wound on these two sections with green D. S. C. wire. Ter¬ 
minals of coils have soldering lugs and are numbered to correspond with enclosed circuit 
diagrams. The transformer is mounted on the rear of the Radio Guild leak-proof 
variable condenser. The resistance of this condenser is so low that it can hardly be 
measured. Wave-length range 220 to 550 meters. Shipping Weight, 1 y 2 lbs. Ship¬ 
ping Weight, coil alone, 12 ozs. 

STOCK NO. 400—Double-Dec R. F. Transformer (complete with AA 
j condenser), each.* «UU 

STOCK NO. 402—Double-Dee Coil (Transformer without con- 
1 denser), each ...«p£.*JV 


iiiiiiimmmmiiiiiimmi 

This special unit, devised by 
Mr. Kenneth Harkness, greatly 
increases the efficiency of a tuned 
radio frequency amplifying re¬ 
ceiver. The transformer has a 
concentrated magnetic field which 
prevents self-oscillation and in¬ 
creases amplification. 

Illlllllllllllllllllllllllllllll 


RADIO FREQUENCY TRANSFORMER 


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ingle Socket! 

Equip your set with the 
Radio Guild socket—there 
is none better. This socket 
is built to endure, built 
to give you lasting satis¬ 
faction. The receptacle is 
wedged into the Quarter- 
inch hard-rubber base by 
a special process and is 
guaranteed to remain firm 
and securely in position 
for the life of your re¬ 
ceiver. Panel or table 
mounting. Base 2% inches 
by 3 Ya inches. Shipping 
Weight % lb. 

STOCK NO. 600— or 
Single Socket.ODC 


For receiving sets employing a com¬ 
bination of three tubes the Triple Socket 
is the most satisfactory and efficient ar¬ 
rangement. All you need to mount this 
socket on the panel of your receiver is 
a screw driver. Three small screws, 
turned into the special brackets, fasten 
the socket permanently to your panel. 
If you prefer table mounting, special 
washers are provided for this purpose. 
SPECIFICATIONS 

Metal parts are nickel-plated and 
highly polished. The engraved terminals 
insure your making correct connections. 
The high Quality Quarter-inch hard rub¬ 
ber base gives perfect insulation and 
strength. Strong contact strips made of 
'i heavy nickel-plated phosphor bronze give 
positive contact with the tube at all 
times and never lose their springiness. 
The tube receptacles are securely wedged 
into the hard rubber base by a special 
process; they can never become loose. Base 3 
inches by 7% inches. Mounting screws spaced 
3 5/16 inches between centers. Shipping weight 
1% lbs. 

STOCK NO. 602—Triple 
Socket. 


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The finest battery on the market. The result of 20 years research and experiment. 

Absolutely guaranteed uniform high voltage, long life. They insure better reception, uniformity 
of discharge, freedom from noises. 

These batteries are made in all standard sizes. If your dealer cannot supply you promptly use 
our order form and we will see that you are supplied from our stock of batteries received fresh from 
the manufacturer each week. 

Stock No. 810—22% volt, medium size; Length, 4% inches; Width, 2% Inches; Height, 2% 

inches; Weight, 1 lb. 6 ozs.$1.90 

Stock No. 812—22% volt, large size; Length, 6% inches; Width, 3% inches; Height, 3 inches; 

Weight. 3 lbs. 14 ozs.$2.50 

Stock No. 814—45 volt. Baby size; Length, 5% inches; Width, 4% inches; Height, 2% inches; 

Weight, 3 lbs. 9 ozs.$4.00 

Stock No. 816—45 volt. Large size; Length, 7% inches; Width, 6% inches; Height, 3 inches; 

Weight, 8% lbs.$5.00 

Stock No. 818—22% volt. Skyscraper size; Length, 2 9/16 inches; Width, 3% inches; Height, 
5% inches; Weight, 2 lbs. 3 ozs.$2.25 


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Use a Diode Tube as rectifier in 
the next receiver you build. It will 

cost you just a fraction of the price 
you would have to pay for a three- 
element tube and its accessories. 
You will not only save money—you 
will gain efficiency, too; especially 
if your receiver is a reflex. The 
Diode is far better than a three- 
element tube as rectifier in a reflex 
circuit. Mr. Kenneth Harkness en¬ 
dorses it for use in the Harkness 
Circuit. 

The Diode is noiseless. It re¬ 
quires no B battery, and will op¬ 
erate on less than half an ampere 
from a single dry cell. With ordinary handling it will burn from 600 
to 1,000 hours. , 

The terminals on the moulded socket base of the Diode are plainly 
marked so that you cannot make a wrong connection. Diameter of 
base—1% in. Total height, tube and socket—3% in. Shipping weight— 
12 oz. 

Stock No. 826—Diode Tube and Socket...-..$2.50 


If Your Dealer Cannot Supply You Promptly 


pls^Order Form On Last Pag'e 1 






















Condenser 


and double the lstren< 
of your signals 


g Maximum Cap.—.00032 MF. 

^ Minimum Cap.—.00001 MF. 
g Resistance (hi. frea.)—.4 ohm. 
i Phase diff. (hi. freq.)— 2 mms. 
| Plates—7 rotor, 8 stator, 
g End Plates—Nickeled Brass, 
g Revolves on ball-bearing, 
g Tapped holes on rear plate for 
g mounting coil. „ . ,, 

| Width, 3% inches; Height 
g (plates open), 3 inches. 

| Shipping Weight, 1% lbs. 

* 


Electrical 
fc. Losses 


If you could see the losses which take place in 
ordinary variable condensers you would understand 
why your signals are sometimes weak and feeble when 
they should be strong and clear. , 

The engineers of the Radio Guild have now de¬ 
signed a condenser in which all losses are reduced to 
an absolute minimum. The resistance of this con¬ 
denser is so low that it actually increases the signal 
strength and selectivity of a receiver from 50 to 100 
per cent 

The ordinary type of condenser is made up of indi¬ 
vidual plates separated by washers. This method of 
construction produces high resistance. The resistance 
of the Radio Guild condenser, however, is astonishingly 
low because all the movable plates are die-cast in one 
solid piece and all the stationary plates are cast in, 
another solid piece of metal. The movable casting is 
electrically connected to the rotor terminal by a special 
spring pigtail which positively eliminate the resistance of friction contacts used in 
other condensers. Other losses caused by leakage and dielectric absorption are prac¬ 
tically reduced to zero in the Guild condenser. 

The Radio Guild condenser is intended for use in any standard circuit to take the 
place of the average 23-plate condenser. Although its maximum capacity is lower 
than that of the average 23-plate condenser its minimum capacity is also much 
lower. Consequently it can be used to cover the same wave length band—and with 
far ‘greater efficiency. 

See this condenser at your dealers. An examination will reveal its striking elec¬ 
trical superiority, its mechanical strength and durability, its handsome appearance and 
perfect proportions. If your dealer cannot supply you, use our order form and we will 
be glad to serve you by return mail. Every Radio Guild condenser is fully 
guaranteed. 

STOCK NO. 110—Leak-proof Condenser (.00032 Mf.). . $4.50 


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OrderBlank S&wsss®#?' 

V/<UCi ^ iU/ New York Citij 


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On request we shall be glad to supply you 

with a quantity of these Order Blanks. 


W ri te qou r order on t his Order BlanKand itwill 
be given Preferred Attention the dag we receive it 


[Satisfaction Guaranteed orVburMoneijRefunded 


















































































































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Country of origin and production U. S A. 


Printed by Steinberg Press, Inc., 409 Pearl St., New York, U. S. A- 




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